Restore CI, including Python 2.7

[maxbelanger]

• Restores CI to a green state to aid in landing other changes.
• Python 2.7 is still covered, but only on Ubuntu and excluding PyPy. Changes are made to requirements.txt et al to ensure this works.
• Fixes an integration test breaking due to backend changes, to be investigated separately.

[april]

Looks good to me, with the caveat that I am relying on the CI/CD pipeline to test this for me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.26%. Comparing base (487793d) to head (8cca9df).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
+ Coverage   63.69%   64.26%   +0.56%     
==========================================
  Files          31       31              
  Lines       52609    52609              
  Branches     3841     3841              
==========================================
+ Hits        33510    33807     +297     
+ Misses      19064    18641     -423     
- Partials       35      161     +126     

Flag	Coverage Δ
integration	64.17% <ø> (?)
unit	63.69% <ø> (ø)

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