Add .python-version to .gitignore

[d-ryzhykau]

What do these changes do?

Adds .python-version (which is used to specify pyenv version) to the list of ignored files.

Are there changes in behavior for the user?

As a developer of the project, I can specify python version for pyenv that is configured for the project.

[codecov]

Codecov Report

Merging #424 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #424   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files           9        9           
  Lines        1180     1180           
  Branches      172      172           
=======================================
  Hits         1089     1089           
  Misses         64       64           
  Partials       27       27

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