Drop Python 2.x and 3.6 support

[aaront]

• Drops support for Python 2.x and removes shims
• Bumps minimum required Python 3 version to 3.7 and greater

[codecov-commenter]

Codecov Report

Merging #12 (8e80d1b) into master (5407f7c) will decrease coverage by 0.21%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   81.46%   81.25%   -0.22%     
==========================================
  Files           6        6              
  Lines         383      384       +1     
  Branches       77       72       -5     
==========================================
  Hits          312      312              
  Misses         53       53              
- Partials       18       19       +1     

Impacted Files	Coverage Δ
mygeotab/ext/entitylist.py	59.15% <50.00%> (ø)
mygeotab/serializers.py	53.57% <50.00%> (ø)
mygeotab/api.py	90.32% <66.66%> (-1.13%)	⬇️
mygeotab/api_async.py	86.74% <100.00%> (ø)

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