Fix Python 3.8 SyntaxWarning: "is not" with a literal

[ensarkarabudak]

/usr/lib/python3/dist-packages/gitsome/github.py:792: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  ('/' + language if language is not 'overall' else '') +

This is a new warning added in Python 3.8. From the release notes:

The compiler now produces a SyntaxWarning when identity checks (is and is not) are used with certain types of literals (e.g. strings, numbers). These can often work by accident in CPython, but are not guaranteed by the language spec. The warning advises users to use equality tests (== and !=) instead. (Contributed by Serhiy Storchaka in bpo-34850.)

[codecov-io]

Codecov Report

Merging #182 (93a60b6) into master (d7c57ab) will decrease coverage by 41.80%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #182       +/-   ##
===========================================
- Coverage   95.09%   53.28%   -41.81%     
===========================================
  Files          34       86       +52     
  Lines        2119     6585     +4466     
===========================================
+ Hits         2015     3509     +1494     
- Misses        104     3076     +2972     

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