Add checking of Python version to the CLI tool, tc

[martinholmer]

No change in tax logic.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (e35dae3) to head (e662c81).
Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2827   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          13       13           
  Lines        2613     2615    +2     
=======================================
+ Hits         2598     2600    +2     
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)