Enhance GitHUB CI Workflow

[pepoluan]

Speedup of #323 + many modernizations, such as:

• Add pypy3.8
• Exclude pypy3.6 on Windows (no longer available)
• Remove 3.6 (end of life already -- let it rest in peace)
• Actions use Node 16

And a workaround:

• Skip code coverage when running on pypy3.8 (not sure why it doesn't hit 100%)

Now all steps are green and runs in acceptable time, as can be seen in CI #451

If merged this closes #316

[pepoluan]

When this is merged, we can again be sure that our CI no longer raises false alarm, and I can package & tag 1.4.3.

[codecov[bot]]

Codecov Report

Merging #324 (752fae9) into master (c78fc95) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #324   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1691      1696    +5     
  Branches       314       310    -4     
=========================================
+ Hits          1691      1696    +5     

Impacted Files	Coverage Δ
aiosmtpd/smtp.py	100.00% <100.00%> (ø)
aiosmtpd/handlers.py	100.00% <0.00%> (ø)
aiosmtpd/controller.py	100.00% <0.00%> (ø)
aiosmtpd/proxy_protocol.py	100.00% <0.00%> (ø)

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[pepoluan]

If no objections within say 2x24 hours I will squamerge, then package the code so far into 1.4.3.

Hopefully can answer the "severe/critical" blocker Debian maintainer are facing right now (as mentioned in #322 )

[pepoluan]

Testing with the latest commit 752fae9 : All is green

Can squamerge this.

After this is merged: Bump the version, update NEWS.rst, release new version here and in PyPI.