chore: drop python 3.6

[mayeut]

Time to drop Python 3.6 support.

pypinfo -l 25 --percent 'pybase64==1.3.*' pyversion

python_version	percent	download_count
3.10	62.12%	336,171
3.9	12.38%	67,003
3.8	12.05%	65,224
3.11	8.05%	43,543
3.7	2.95%	15,954
3.12	2.12%	11,481
3.6	0.31%	1,697
3.3	0.01%	50
3.4	0.00%	4
2.7	0.00%	3
3.13	0.00%	2
Total		541,132

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f80c91f) 93.32% compared to head (ae18c90) 93.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #670      +/-   ##
==========================================
+ Coverage   93.32%   93.35%   +0.03%     
==========================================
  Files           8        8              
  Lines        1063     1069       +6     
  Branches      289      289              
==========================================
+ Hits          992      998       +6     
  Misses         35       35              
  Partials       36       36              

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