Fix processing of scheduled sheet coupon assignment tasks

[mbertrand]

Pre-Flight checklist

• Testing
  • Code is tested
  • Changes have been manually tested

What are the relevant tickets?

Closes #2456

What's this PR do?

Stops treating celery task metadata kwargs as a serialized string, because it is now a proper dict (maybe as a result of a prior celery upgrade?)

How should this be manually tested?

In a shell run the following:

from sheets.tasks import schedule_coupon_assignment_sheet_handling, _get_scheduled_assignment_task_ids
schedule_coupon_assignment_sheet_handling.delay("1xMmUnWm95uyMKXQm-_262tARY0uGYLcSay0qKhud1Zo")
print(_get_scheduled_assignment_task_ids("1xMmUnWm95uyMKXQm-_262tARY0uGYLcSay0qKhud1Zo"))

On this branch, the print statement should show a list with one value. On master branch, it will throw an error (AttributeError: 'dict' object has no attribute 'replace')

[codecov-commenter]

Codecov Report

Merging #2565 (f55e166) into master (ac480c2) will increase coverage by 8.66%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #2565      +/-   ##
==========================================
+ Coverage   88.34%   97.00%   +8.66%     
==========================================
  Files         324      150     -174     
  Lines       15346     4773   -10573     
  Branches     1133        0    -1133     
==========================================
- Hits        13557     4630    -8927     
+ Misses       1524      143    -1381     
+ Partials      265        0     -265     

Impacted Files	Coverage Δ
sheets/tasks.py
users/management/commands/unblock_users.py
ecommerce/conftest.py
b2b_ecommerce/admin.py
mail/urls.py
courseware/factories.py
courses/serializers.py
courseware/constants.py
courses/models.py
compliance/apps.py
... and 164 more

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[rachellougee]

Code and tests are good for me. No AttributionError with this branch