Drop Python 3.6 support

[asvetlov]

No description provided.

[codecov]

Codecov Report

Merging #672 (bd11dec) into master (7a7f239) will increase coverage by 3.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #672      +/-   ##
==========================================
+ Coverage   96.61%   99.72%   +3.10%     
==========================================
  Files           4        4              
  Lines         739      717      -22     
  Branches      168      162       -6     
==========================================
+ Hits          714      715       +1     
+ Misses         23        2      -21     
+ Partials        2        0       -2     

Flag	Coverage Δ
unit	99.58% <100.00%> (+2.96%)	⬆️

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Impacted Files	Coverage Δ
yarl/_url.py	99.63% <100.00%> (+3.66%)	⬆️
yarl/_quoting.py	100.00% <0.00%> (+20.00%)	⬆️

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