Fix pydantic json_encoders deprecation in BaseModel

[aagbsn]

Fixes deprecation warning about json_encoders

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (75664aa) to head (db00e53).
⚠️ Report is 1 commits behind head on master.

❌ Your project check has failed because the head coverage (94.87%) is below the target coverage (95.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1227      +/-   ##
==========================================
+ Coverage   92.42%   94.87%   +2.44%     
==========================================
  Files          86       58      -28     
  Lines        8599     5231    -3368     
  Branches      509      212     -297     
==========================================
- Hits         7948     4963    -2985     
+ Misses        541      237     -304     
+ Partials      110       31      -79     

Flag	Coverage Δ
ooniauth	100.00% <ø> (ø)
oonifindings	97.32% <ø> (ø)
oonimeasurements	?
ooniprobe	91.28% <ø> (+0.94%)	⬆️
oonirun	98.85% <ø> (ø)

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[hellais]

I think we should add a few more tests for this specific use-case. I am also seeing that some of the existing tests are failing and it's unclear if some downstream consumers might break as a result of this.

One thing we should be extra careful of is that only one serializer can be defined per field, so I'm not sure if by defining a catch all field serialiser this might impact how subclasses of this base class which use serialisers work.

Some additional tests and validation for that could help address this concern as well.

[aagbsn]

Added regression tests to confirm that this PR does break backwards compatibility, and then fixed the regression.

[aagbsn]

There may be some additional endpoints that need tests to ensure no other regressions exist.

[aagbsn]

On this branch I rebased additional date format regression tests to verify that the services ooniauth, oonifindings, and oonirun were not affected by this change
https://github.com/ooni/backend/tree/test_datetime_regression