[Bug in Google style] Return type parsed incorrectly with a | b style

[jsh9]

Hi, I'm the developer of pydoclint, which uses docstring_parser to parse Google-style and Sphinx-style docstrings.

In this issue (jsh9/pydoclint#66), we found that docstring_parser encountered an issue if the return type is something like a | b.

For example:

Returns:
    Foo | int: Some description.

But if the return type is Union[a, b], the parsing is correct.