Fix crash when using f-strings

[kasium]

Closes #640

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

[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 #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