Remove Python 3.6 support

[ahopkins]

Per conversation on forums v21.3 will drop Python 3.6 support. 20.12LTS will of course continue to support it.

[codecov]

Codecov Report

Merging #2036 (8f3e076) into master (8f8c00a) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2036      +/-   ##
==========================================
- Coverage   92.13%   92.06%   -0.07%     
==========================================
  Files          33       33              
  Lines        3104     3102       -2     
  Branches      542      542              
==========================================
- Hits         2860     2856       -4     
- Misses        165      166       +1     
- Partials       79       80       +1     

Impacted Files	Coverage Δ
sanic/server.py	87.95% <100.00%> (-0.07%)	⬇️
sanic/__main__.py	75.00% <0.00%> (-3.85%)	⬇️

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[ahopkins]

This pull request has been mentioned on Sanic Community Discussion. There might be relevant details there:

https://community.sanicframework.org/t/dropping-python-3-6/793/6

[ashleysommer]

Great.

[ahopkins]

Thanks. This will make it easier to clean up some of the other PRs.