Bump python versions, prepare new release

[jdebacker]

This PR increases the Python versions that Tax-Brain is tested against in CI testing and also the versions for the Python package.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.17%. Comparing base (dd8458d) to head (23e3fa4).
⚠️ Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
setup.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   79.50%   79.17%   -0.33%     
==========================================
  Files          23       23              
  Lines        1947     1974      +27     
==========================================
+ Hits         1548     1563      +15     
- Misses        399      411      +12     

Flag	Coverage Δ
unittests	79.17% <75.00%> (-0.33%)	⬇️

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