[Python] Align with PEP8, PEP257

Description

Since #3041 it seems the templates have drifted a bit from PEP8 guidelines. A couple examples that jumped at me:

• Some inconsistencies with acronym capitalization (ex: RESTClientObject and ApiClient) ref
• Lots of one-line docstrings taking up 3 lines ref
• Docstrings on property setters that are ignored by help() and tools like Sphinx: example from Python docs

Not in PEP8, but there are a few instances where default values are set in __init__() using property setters before the underlying attribute is declared, which will throw an AttributeError if that initial assignment isn't made.

These are pretty low-key, but it'll help keep the linters quiet

Swagger-codegen version

2.2.0 and 2.3.0

Related issues

#3041

Suggest a Fix

I can clean up the templates a bit and create a PR (I did quite a bit of it already)