fix: using get_course for course lookup in modulestore

[jszewczulak]

TL;DR - As it turns out to do course block lookup using modulestore and CourseKey we need to call get_course method, get_item gives error when passing CourseKey

Developer Checklist

• Reviewed the release process
• Translations and JS/SASS compiled
• Bumped version number in openassessment/__init__.py and package.json

Testing Instructions

[ How should a reviewer test this PR? ]

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• I've done a visual code review
• I've tested the new functionality

FYI: @openedx/content-aurora

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (9f105e5) 94.93% compared to head (d2d149b) 94.93%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2064   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         183      183           
  Lines       18621    18623    +2     
  Branches     1704     1704           
=======================================
+ Hits        17677    17679    +2     
  Misses        712      712           
  Partials      232      232           

Flag	Coverage Δ
unittests	94.93% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
openassessment/__init__.py	100.00% <100.00%> (ø)
...nt/workflow/test/test_workflow_batch_update_api.py	99.19% <100.00%> (+<0.01%)	⬆️
...enassessment/workflow/workflow_batch_update_api.py	80.00% <100.00%> (ø)

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