Assertion check fails on syntactically-valid Python code

Originally reported here: https://github.com/wemake-services/wemake-python-styleguide/issues/1599#issuecomment-703526278

There is a problem with the case of ast.Tuple containing anything other than class name or ast.Attribute: source code that uses such constructs in except handler actually trips assertion checks inside flake8-bugbear visitors:

Test file:

try:
    pass
except (ValueError, "oops"):
    print(123)

Error output:

❯ flake8 1.py
Traceback (most recent call last):
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/bin/flake8", line 10, in <module>
    sys.exit(main())
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/main/cli.py", line 22, in main
    app.run(argv)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/main/application.py", line 360, in run
    self._run(argv)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/main/application.py", line 348, in _run
    self.run_checks()
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/main/application.py", line 262, in run_checks
    self.file_checker_manager.run()
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/checker.py", line 325, in run
    self.run_serial()
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/checker.py", line 309, in run_serial
    checker.run_checks()
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/checker.py", line 589, in run_checks
    self.run_ast_checks()
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/flake8/checker.py", line 496, in run_ast_checks
    for (line_number, offset, text, _) in runner:
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 36, in run
    visitor.visit(self.tree)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 156, in visit
    super().visit(node)
  File "/Users/lensvol/.pyenv/versions/3.7.4/lib/python3.7/ast.py", line 262, in visit
    return visitor(node)
  File "/Users/lensvol/.pyenv/versions/3.7.4/lib/python3.7/ast.py", line 270, in generic_visit
    self.visit(item)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 156, in visit
    super().visit(node)
  File "/Users/lensvol/.pyenv/versions/3.7.4/lib/python3.7/ast.py", line 262, in visit
    return visitor(node)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 292, in visit_Try
    self.generic_visit(node)
  File "/Users/lensvol/.pyenv/versions/3.7.4/lib/python3.7/ast.py", line 270, in generic_visit
    self.visit(item)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 156, in visit
    super().visit(node)
  File "/Users/lensvol/.pyenv/versions/3.7.4/lib/python3.7/ast.py", line 262, in visit
    return visitor(node)
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 165, in visit_ExceptHandler
    names = [_to_name_str(e) for e in node.type.elts]
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 165, in <listcomp>
    names = [_to_name_str(e) for e in node.type.elts]
  File "/Users/lensvol/Library/Caches/pypoetry/virtualenvs/wemake-python-styleguide-py3.7/lib/python3.7/site-packages/bugbear.py", line 130, in _to_name_str
    assert isinstance(node, ast.Attribute)
AssertionError