Add setuptools-rust to dev

[puddly]

mypy complains if setuptools_rust isn't installed (#128) on Linux. It isn't used as a build backend just because it's a dependency and the package is pure-Python.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.33%. Comparing base (029738e) to head (f97be54).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #133      +/-   ##
==========================================
+ Coverage   94.31%   94.33%   +0.02%     
==========================================
  Files          25       25              
  Lines        4345     4345              
==========================================
+ Hits         4098     4099       +1     
+ Misses        247      246       -1     

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