Switch Tax-Calculator code to being compatible with Python 3.6 only

[martinholmer]

The next Tax-Calculator release will be 0.21.0 and the release notes for that version will include an API Change that says it is compatible with only Python 3.6.

Incompatibilities in the pandas library for Python 2.7 and Python 3.6 make it extremely difficult to make Tax-Calculator code and Cookbook recipes be compatible with both Python 2.7 and Python 3.6. We've been warning for many months that support for Python 2.7 was ending soon and that Tax-Calculator users should upgrade to Python 3.6 as soon as possible. The 0.20.3 release, which is compatible with Python 2.7, will remain available to users who have not upgraded to Python 3.6.

[codecov-io]

Codecov Report

Merging #2058 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2058      +/-   ##
==========================================
- Coverage   99.97%   99.97%   -0.01%     
==========================================
  Files          16       16              
  Lines        3530     3515      -15     
==========================================
- Hits         3529     3514      -15     
  Misses          1        1

Impacted Files	Coverage Δ
taxcalc/behavior.py	100% <ø> (ø)	⬆️
taxcalc/growmodel.py	98.18% <ø> (-0.04%)	⬇️
taxcalc/taxcalcio.py	100% <100%> (ø)	⬆️
taxcalc/calculate.py	100% <100%> (ø)	⬆️
taxcalc/decorators.py	100% <100%> (ø)	⬆️
taxcalc/records.py	100% <100%> (ø)	⬆️
taxcalc/policy.py	100% <100%> (ø)	⬆️
taxcalc/utils.py	100% <100%> (ø)	⬆️
taxcalc/growfactors.py	100% <100%> (ø)	⬆️
taxcalc/parameters.py	100% <100%> (ø)	⬆️
... and 1 more

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