Add flake8-docstrings to dev requirements

[GenevieveBuckley]

Add flake8-docstrings to dev requirements, so the pre-commit hook also catches docstring issues.

I was briefly confused earlier today when the qa(lint) job failed on CI for my pull request, because I had installed the pre-commit hook and flake8 magicgui passed on my local machine. Turns out that this wasn't checking any of the docstrings locally, because I did not have flake8-docstrings installed into my development environment. This PR adds flake8-docstrings to requirements/dev.txt.

[codecov]

Codecov Report

Merging #39 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files          14       14           
  Lines         934      934           
=======================================
  Hits          895      895           
  Misses         39       39           

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