Add mypy

[ashleyzhang01]

fixing type errors, getting rid of anys
todo after fixing: schemas, unused params, organize types

[codecov]

Codecov Report

Attention: Patch coverage is 94.59746% with 51 lines in your changes missing coverage. Please review.

Project coverage is 90.94%. Comparing base (96a605c) to head (f6034f9).
Report is 10 commits behind head on add-type-safety.

Files with missing lines	Patch %	Lines
backend/sublet/views.py	87.64%	11 Missing ⚠️
backend/pennmobile/admin.py	56.25%	7 Missing ⚠️
backend/portal/permissions.py	84.21%	6 Missing ⚠️
backend/sublet/serializers.py	90.90%	5 Missing ⚠️
...nndata/management/commands/get_fitness_snapshot.py	63.63%	4 Missing ⚠️
backend/gsr_booking/admin.py	70.00%	3 Missing ⚠️
backend/gsr_booking/models.py	94.91%	3 Missing ⚠️
backend/gsr_booking/api_wrapper.py	96.55%	2 Missing ⚠️
backend/utils/email.py	81.81%	2 Missing ⚠️
backend/dining/api_wrapper.py	90.90%	1 Missing ⚠️
... and 7 more

Additional details and impacted files

@@                 Coverage Diff                 @@
##           add-type-safety     #322      +/-   ##
===================================================
+ Coverage            90.86%   90.94%   +0.07%     
===================================================
  Files                   63       64       +1     
  Lines                 2661     2903     +242     
===================================================
+ Hits                  2418     2640     +222     
- Misses                 243      263      +20     

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