Add support for Python 3.9 & boilerplate

[ewjoachim]

• Add support to Python 3.9
• Switch to Poetry
• Switch to a new folder named scripts that contains small scripts that reference usual project commands (instead of tox)
• Speed up CI

Successful PR Checklist:

• Tests
  • (not applicable?)
• Documentation
  • (not applicable?)

[codecov]

Codecov Report

Merging #397 (0e09765) into master (8dbcd86) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

❗ Current head 0e09765 differs from pull request most recent head 8356e93. Consider uploading reports for the commit 8356e93 to get more accurate results

@@             Coverage Diff             @@
##            master     #397      +/-   ##
===========================================
- Coverage   100.00%   99.87%   -0.13%     
===========================================
  Files           28       28              
  Lines         1626     1612      -14     
  Branches       177      175       -2     
===========================================
- Hits          1626     1610      -16     
- Misses           0        1       +1     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
procrastinate/contrib/django/migrations_magic.py	100.00% <ø> (ø)
procrastinate/signals.py	100.00% <ø> (ø)
procrastinate/metadata.py	100.00% <100.00%> (ø)
procrastinate/schema.py	100.00% <100.00%> (ø)
procrastinate/sql/__init__.py	100.00% <100.00%> (ø)
procrastinate/worker.py	98.80% <0.00%> (-1.20%)	⬇️
procrastinate/periodic.py	100.00% <0.00%> (ø)
procrastinate/aiopg_connector.py	100.00% <0.00%> (ø)

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[mgu]

🚀

[elemoine]

Looks fantastic! Thanks for this.