build: Remove _version.py from bump2version control

[matthewfeickert]

* _version.py is generated from setuptools_scm and so does not need to
be bumped by bump2version

[codecov-commenter]

Codecov Report

Merging #20 (7dbc05f) into master (8c26582) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #20   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files           7        7           
  Lines         482      482           
  Branches       82       82           
=======================================
  Hits          456      456           
  Misses         23       23           
  Partials        3        3           

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