Fix crash when using f-strings

[kasium]

Closes #640

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[sigmavirus24]

This prevents the crash but it misleads people and will lead to big reports of "but there's a docstring there" because it doesn't inform people that the f string is invalid and cannot be a docstring. I would not accept this PR as is

[kasium]

@sigmavirus24 Reason for this is that in https://github.com/PyCQA/pydocstyle/pull/381 the following was suggested

Mistakenly using an f-string for a docstring does not cause a syntax error, so we shouldn't raise a ParseError here. Instead, I suggest we rename this as eval_docstring and just return an empty string if we find an f-string.

I'm fine adding a new error code saying that an f-string is invalid, but this idea was dropped before

[sigmavirus24]

The idea for adding a new error code was not dropped. The PR lost steam due to people being busy. The new error code is correct behavior. How we achieve that is up for debate