tolerate square brackets in return type annotations

[eanopolsky]

Currently, sphinx-doc does not create docstrings when return type annotations contain square brackets.

Example:

from typing import Optional


def test(a, b) -> Optional[str]:
    if a == b:
        return "foo"
    else:
        return None

Steps to reproduce:

1. Create the example file as specified above.
2. Move the point into the body of the function.
3. Run M-x sphinx-doc RET.

Expected results:
A docstring should appear.

Actual results:
The buffer is unchanged and the following message is logged: Failed to parse function definition ’def test(a, b) -> Optional[str]:’.

This PR modifies the return type regex to tolerate square brackets in return type annotations and adds a corresponding test.