Stop support for python < 3.6

[azmeuk]

I also applied pyupgrade --py36-plus to automatically migrate the code.

[codecov-commenter]

Codecov Report

Merging #416 into master will decrease coverage by 1.71%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master     #416      +/-   ##
==========================================
- Coverage   99.68%   97.97%   -1.72%     
==========================================
  Files          18       18              
  Lines         949      936      -13     
  Branches       74       73       -1     
==========================================
- Hits          946      917      -29     
- Misses          3       17      +14     
- Partials        0        2       +2     

Impacted Files	Coverage Δ
flask_wtf/_compat.py	92.30% <50.00%> (-7.70%)	⬇️
flask_wtf/csrf.py	98.11% <100.00%> (+0.01%)	⬆️
flask_wtf/file.py	91.66% <100.00%> (-8.34%)	⬇️
flask_wtf/form.py	100.00% <100.00%> (ø)
flask_wtf/i18n.py	100.00% <100.00%> (ø)
flask_wtf/recaptcha/fields.py	100.00% <100.00%> (ø)
flask_wtf/recaptcha/validators.py	100.00% <100.00%> (ø)
flask_wtf/recaptcha/widgets.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_i18n.py	100.00% <100.00%> (ø)
... and 3 more

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