Fix CI python version

[ewjoachim]

Fix CI python version

Successful PR Checklist:

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

[codecov]

Codecov Report

Merging #435 (cd2ae23) into master (03cfbd4) will increase coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #435      +/-   ##
===========================================
+ Coverage   99.87%   100.00%   +0.12%     
===========================================
  Files          30        30              
  Lines        1630      1635       +5     
  Branches      178       178              
===========================================
+ Hits         1628      1635       +7     
+ Misses          1         0       -1     
+ Partials        1         0       -1     

Impacted Files	Coverage Δ
procrastinate/periodic.py	100.00% <0.00%> (ø)
procrastinate/sql/__init__.py	100.00% <0.00%> (ø)
procrastinate/aiopg_connector.py	100.00% <0.00%> (ø)
procrastinate/worker.py	100.00% <0.00%> (+1.29%)	⬆️

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[ewjoachim]

Haha 😭
I just discovered a typo in the CI that meant we didn't actually test on multiple Python versions. And now it says there's a test that doesn't pass on py 3.6 😓

[k4nar]

😅