Add py39 configs and prepare to drop py36

[izn]

Hey.

This is my first PR here. 🎉

What I did

1. Create a new action to run py39 in GitHub Actions;
2. Replace py36 by py38 for all CI actions;
3. Force black to use py38.

Why I didn't drop 3.6?

Well, I'm not sure if we really need to. My suggestion is to keep an isolated GitHub action for this version until it becomes obsolete.

How to verify it

Check GitHub Actions.

Why this build is 🔴?

Python 3.9 build is breaking due to davesque/blake2b-py installation.

The requirement to install Rust is related to ethereum/blake2b-py#2 - as soon as this is fixed upstream we plan to roll out official support for Python 3.9.

We must wait ethereum/blake2b-py#2 to rebuild.

Related issue

#2182

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #2233 (0d5c798) into master (a222091) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2233      +/-   ##
==========================================
+ Coverage   85.37%   85.40%   +0.03%     
==========================================
  Files          84       84              
  Lines        8552     8552              
  Branches     2066     2066              
==========================================
+ Hits         7301     7304       +3     
+ Misses        745      743       -2     
+ Partials      506      505       -1     

Impacted Files	Coverage Δ
vyper/cli/vyper_json.py	85.20% <0.00%> (+0.40%)	⬆️
vyper/ast/validation.py	72.09% <0.00%> (+4.65%)	⬆️

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[iamdefinitelyahuman]

Hi, and thanks your contribution!

The requirement to install rust is related to ethereum/blake2b-py#2 - as soon as this is fixed upstream we plan to roll out official support for Python 3.9. The required code changes were handled in #2225 so I think this is the only remaining blocker.

It might be better if you remove rust installation from the py39-core job. When the upstream dep is dealt with, this PR should be able to pass CI without installing rust, and then it will be ready to merge.

[izn]

@iamdefinitelyahuman awesome, thanks for your quick response. I've pushed a new commit and updated the PR's description. :)

[fubuloubu]

Rebase on master

Py 3.9 support was added to blake2s-py per v0.1.4, so update the pin for that package and this should be ready to merge!

[izn]

@fubuloubu rebased. ✔️ (cc @iamdefinitelyahuman)