Updating README - Post-Update Hook

This is the specific error for the current code. Windows doesn't like the code being sent to it.

'.' is not recognized as an internal or external command,
operable program or batch file.
shell returned 1

function! BuildYCM(info)
  " info is a dictionary with 3 fields
  " - name:   name of the plugin
  " - status: 'installed', 'updated', or 'unchanged'
  " - force:  set on PlugInstall! or PlugUpdate!
  if a:info.status == 'installed' || a:info.force
    !./install.py
  endif
endfunction

Plug 'ycm-core/YouCompleteMe', { 'do': function('BuildYCM') }

Successfully installs using the correct version of Python.
Should probably be updated for Linux as well.

function! BuildYCM(info)
  " info is a dictionary with 3 fields
  " - name:   name of the plugin
  " - status: 'installed', 'updated', or 'unchanged'
  " - force:  set on PlugInstall! or PlugUpdate!
  if a:info.status == 'installed' || a:info.force
    if has('win32')
      let py_version = py3eval('sys.version.split()[0].replace(".", "")')
      let py_version = py_version[:0] . '.' . py_version[1:]
      let command = printf('!py -%s %s', py_version, shellescape(expand('.\install.py')))
      execute(command)
    else
      !./install.py
    endif
  endif
endfunction

Plug 'ycm-core/YouCompleteMe',
 \ { 'do': function('BuildYCM') }

Successfully installs the correct version on Windows.
It's a bit more verbose (mostly due to the docstring), but I think it's a lot easier to read for most people.

function! BuildYCM(info)
python3 << EOF
""" Post-Update Hook for YouCompleteMe using Vim-Plug.

Limitations
-----------
Only supports Windows and Linux

Arguments
---------
info : 3-dict
    name : str
        Name of the plugin.
    status : {'installed', 'updated', or 'unchanged'}
    force : bool
        Set to True on `PlugInstall!` or `PlugUpdate!`.

"""
from pathlib import Path


info = vim.eval('a:info')
if info['status'] == 'installed' or info['force']:

    py_version = sys.version.split()[0].replace('.', '')
    py_version = py_version[:1] + '.' + py_version[1:]
    os_specific = f'py -{py_version}' if sys.platform == 'win32' else ''
    command = f'!{os_specific} {Path("./install.py")}'

vim.command(command)
EOF
endfunction

Plug 'ycm-core/YouCompleteMe',
 \ { 'do': function('BuildYCM') }

Based on the original script, this solves the issue of Windows vs. Linux, but won't use the proper version.
This may not be an issue with python-x, but I'm not that savvy.

function! BuildYCM(info)
  " info is a dictionary with 3 fields
  " - name:   name of the plugin
  " - status: 'installed', 'updated', or 'unchanged'
  " - force:  set on PlugInstall! or PlugUpdate!
  if a:info.status == 'installed' || a:info.force
    execute('!' . shellescape(expand('./install.py')))
  endif
endfunction

Plug 'ycm-core/YouCompleteMe',
 \ { 'do': function('BuildYCM') }

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Dec 12 2019 13:30:17)
MS-Windows 32-bit GUI version with OLE support
Compiled by mool@tororo
Huge version with GUI.  Features included (+) or not (-):
+acl                +eval               +multi_lang         -tag_any_white
+arabic             +ex_extra           +mzscheme/dyn       +tcl/dyn
+autocmd            +extra_search       +netbeans_intg      -termguicolors
+autochdir          -farsi              +num64              +terminal
+autoservername     +file_in_path       +ole                -termresponse
+balloon_eval       +find_in_path       +packages           +textobjects
-balloon_eval_term  +float              +path_extra         +textprop
+browse             +folding            +perl/dyn           -tgetent
++builtin_terms     -footer             +persistent_undo    +timers
+byte_offset        +gettext/dyn        +popupwin           +title
+channel            -hangul_input       -postscript         +toolbar
+cindent            +iconv/dyn          +printer            +user_commands
+clientserver       +insert_expand      +profile            +vartabs
+clipboard          +job                +python/dyn         +vertsplit
+cmdline_compl      +jumplist           +python3/dyn        +virtualedit
+cmdline_hist       +keymap             +quickfix           +visual
+cmdline_info       +lambda             +reltime            +visualextra
+comments           +langmap            +rightleft          +viminfo
+conceal            +libcall            +ruby/dyn           +vreplace
+cryptv             +linebreak          +scrollbind         -vtp
+cscope             +lispindent         +signs              +wildignore
+cursorbind         +listcmds           +smartindent        +wildmenu
+cursorshape        +localmap           +sound              +windows
+dialog_con_gui     +lua/dyn            +spell              +writebackup
+diff               +menu               +startuptime        -xfontset
+digraphs           +mksession          +statusline         -xim
+directx            +modify_fname       -sun_workshop       +xpm_w32
-dnd                +mouse              +syntax             -xterm_save
-ebcdic             +mouseshape         +tag_binary         
+emacs_tags         +multi_byte_ime/dyn -tag_old_static     
   system vimrc file: "$VIM\vimrc"
     user vimrc file: "$HOME\_vimrc"
 2nd user vimrc file: "$HOME\vimfiles\vimrc"
 3rd user vimrc file: "$VIM\_vimrc"
      user exrc file: "$HOME\_exrc"
  2nd user exrc file: "$VIM\_exrc"
  system gvimrc file: "$VIM\gvimrc"
    user gvimrc file: "$HOME\_gvimrc"
2nd user gvimrc file: "$HOME\vimfiles\gvimrc"
3rd user gvimrc file: "$VIM\_gvimrc"
       defaults file: "$VIMRUNTIME\defaults.vim"
    system menu file: "$VIMRUNTIME\menu.vim"
Compilation: cl -c /W3 /nologo  -I. -Iproto -DHAVE_PATHDEF -DWIN32  -DFEAT_CSCOPE -DFEAT_TERMINAL -DFEAT_SOUND -DFEAT_NETBEANS_INTG -DFEAT_JOB_CHANNEL   -DFEAT_XPM_W32   -DWINVER=0x0501 -D_WIN32_WINNT=0x0501 /MP -DHAVE_STDINT_H /Ox /GL -DNDEBUG /arch:IA32 /Zl /MT -DFEAT_OLE -DFEAT_MBYTE_IME -DDYNAMIC_IME -DFEAT_GUI_MSWIN -DFEAT_DIRECTX -DDYNAMIC_DIRECTX -DFEAT_DIRECTX_COLOR_EMOJI -DDYNAMIC_ICONV -DDYNAMIC_GETTEXT -DFEAT_TCL -DDYNAMIC_TCL -DDYNAMIC_TCL_DLL=\"tcl86t.dll\" -DDYNAMIC_TCL_VER=\"8.6\" -DFEAT_LUA -DDYNAMIC_LUA -DDYNAMIC_LUA_DLL=\"lua53.dll\" -DFEAT_PYTHON -DDYNAMIC_PYTHON -DDYNAMIC_PYTHON_DLL=\"python27.dll\" -DFEAT_PYTHON3 -DDYNAMIC_PYTHON3 -DDYNAMIC_PYTHON3_DLL=\"python36.dll\" -DFEAT_MZSCHEME -I "E:\Racket\include" -DMZ_PRECISE_GC -DDYNAMIC_MZSCHEME -DDYNAMIC_MZSCH_DLL=\"libracket3m_a36fs8.dll\" -DDYNAMIC_MZGC_DLL=\"libracket3m_a36fs8.dll\" -DFEAT_PERL -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DDYNAMIC_PERL -DDYNAMIC_PERL_DLL=\"perl524.dll\" -DFEAT_RUBY -DDYNAMIC_RUBY -DDYNAMIC_RUBY_VER=24 -DDYNAMIC_RUBY_DLL=\"msvcrt-ruby240.dll\" -DFEAT_HUGE /Fd.\ObjGXOULYHTRZi386/ /Zi
Linking: link  /nologo /opt:ref /LTCG:STATUS oldnames.lib kernel32.lib advapi32.lib shell32.lib gdi32.lib  comdlg32.lib ole32.lib netapi32.lib uuid.lib /machine:i386 gdi32.lib version.lib   winspool.lib comctl32.lib advapi32.lib shell32.lib netapi32.lib  /machine:i386  libcmt.lib oleaut32.lib user32.lib  /nodefaultlib:lua53.lib  /STACK:8388608  /nodefaultlib:python27.lib /nodefaultlib:python36.lib   "E:\ActiveTcl\lib\tclstub86.lib" winmm.lib WSock32.lib xpm\x86\lib-vc14\libXpm.lib /PDB:gvim.pdb -debug