Warn about invalid string type for docstring

[orsinium]

Rule request

Thesis

F-strings and bytes cannot be used as a docstring (raw strings is ok). For example:

def ab(a, b):
  f"""oh hi
  """
  return a+b

In the example above, neither inspect.getdoc nor ast.get_docstring will interpret the f-string as a docstring.

Also, get_docstring is a good reference for the check implementation: https://github.com/python/cpython/blob/3.8/Lib/ast.py#L236-L259

Reasoning

It's a bug. Once I tried to use an f-string-based docstring to reuse something inside because we use docstrings to generate OpenAPI spec. Turned out it doesn't work.

[sobolevn]

This is a duplicate of PyCQA/pydocstyle#381