Drop support for EOL Python 2.7 and 3.5

[hugovk]

Fixes #36.

[codecov]

Codecov Report

Merging #68 into master will increase coverage by 0.54%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   86.66%   87.20%   +0.54%     
==========================================
  Files           4        2       -2     
  Lines        3044     1485    -1559     
==========================================
- Hits         2638     1295    -1343     
+ Misses        406      190     -216     

Flag	Coverage Δ
#GHA_Ubuntu	87.20% <78.57%> (+0.54%)	⬆️
#GHA_Windows	87.20% <78.57%> (+0.54%)	⬆️
#GHA_macOS	87.20% <78.57%> (+0.54%)	⬆️

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Impacted Files	Coverage Δ
prettytable.py	82.24% <66.66%> (+0.45%)	⬆️
prettytable_test.py	100.00% <100.00%> (+0.70%)	⬆️
D:/a/prettytable/prettytable/prettytable_test.py
D:/a/prettytable/prettytable/prettytable.py

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