feat: Allow for configurable Stripe keys in dev

[scott-codecov]

These are fake values that need to be overridden in local development. Reading from the config allows us to instead specify these keys in the Codecov install YAML locally.

The tests depend on some non-null value there so I just put "default" as the default.

[codecov]

Codecov Report

Merging #36 (822d120) into main (803dc0c) will not change coverage.
The diff coverage is 100.00%.

@@          Coverage Diff          @@
##            main     #36   +/-   ##
=====================================
  Coverage   95.26   95.26           
=====================================
  Files        691     691           
  Lines      14622   14622           
=====================================
  Hits       13929   13929           
  Misses       693     693           

Flag	Coverage Δ
unit	95.20% <100.00%> (ø)
unit-latest-uploader	95.20% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
codecov/settings_dev.py	100.00% <100.00%> (ø)

[hootener]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (803dc0c) 95.20% compared to head (822d120) 95.20%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #36   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files         578      578           
  Lines       14451    14451           
=======================================
  Hits        13758    13758           
  Misses        693      693           

Flag	Coverage Δ
unit	95.20% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
codecov/settings_dev.py	100.00% <100.00%> (ø)

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