Mypy backlog cleanup

[JackUrb]

Overview

Running through all of the mypy issues, as an initial start to #598 and to close some of the nearby issues. Mostly involved just telling mypy what was really going on in certain places, but actually did clarify logic and catch a minor bug in onboarding initialization.

[codecov-commenter]

Codecov Report

Merging #601 (8c07bc6) into main (b009357) will decrease coverage by 0.17%.
The diff coverage is 55.78%.

@@            Coverage Diff             @@
##             main     #601      +/-   ##
==========================================
- Coverage   66.46%   66.28%   -0.18%     
==========================================
  Files          82       83       +1     
  Lines        7562     7605      +43     
==========================================
+ Hits         5026     5041      +15     
- Misses       2536     2564      +28     

Impacted Files	Coverage Δ
...ueprints/static_html_task/static_html_blueprint.py	41.79% <0.00%> (-1.96%)	⬇️
...prints/static_react_task/static_react_blueprint.py	56.52% <0.00%> (-3.95%)	⬇️
...isto/abstractions/providers/mock/mock_datastore.py	53.92% <ø> (-1.32%)	⬇️
...sto/abstractions/providers/mturk/mturk_provider.py	48.75% <0.00%> (ø)
...ephisto/abstractions/providers/mturk/mturk_unit.py	15.93% <0.00%> (-0.27%)	⬇️
...histo/abstractions/providers/mturk/mturk_worker.py	32.96% <0.00%> (ø)
mephisto/abstractions/test/utils.py	81.81% <ø> (ø)
mephisto/data_model/assignment.py	58.13% <0.00%> (-0.46%)	⬇️
mephisto/data_model/task.py	45.20% <0.00%> (-0.63%)	⬇️
mephisto/operations/utils.py	36.36% <ø> (+0.32%)	⬆️
... and 26 more

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[pringshia]

Nicely done