chore: add support for Django3.2

[arslanashraf7]

What are the relevant tickets?

110

What's this PR do?

Adds support for Django3.2

How should this be manually tested?

• There are some specific changes required in edX platform as mentioned in the Readme as well, So you should checkout a specific branch that I created https://github.com/mitodl/edx-platform/tree/arslan/rapid-response-inc.
• Install this xBlock within the edx-platform
• Test it both with django2 and 3 and make sure it works as intended having backward compatibility as well. More details can be seen in the Repo Readme.
• Verifying the points mentioned in the acceptance criteria of the ticket

Where should the reviewer start?

• Checking the documentation on installing the xBlock & then install it in edx-platform having Django2.

[codecov-commenter]

Codecov Report

Merging #111 (545ae85) into master (041c95c) will decrease coverage by 0.02%. The diff coverage is 100.00%.

:exclamation: Current head 545ae85 differs from pull request most recent head 2d52715. Consider uploading reports for the commit 2d52715 to get more accurate results

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   91.44%   91.41%   -0.03%     
==========================================
  Files          18       18              
  Lines         631      629       -2     
  Branches       43       41       -2     
==========================================
- Hits          577      575       -2     
  Misses         50       50              
  Partials        4        4              

Impacted Files	Coverage Δ
rapid_response_xblock/migrations/0001_initial.py	0.00% <ø> (ø)
...id_response_xblock/migrations/0002_block_status.py	0.00% <ø> (ø)
...d_response_xblock/migrations/0003_rename_fields.py	0.00% <ø> (ø)
rapid_response_xblock/migrations/0004_run.py	0.00% <ø> (ø)
rapid_response_xblock/models.py	91.30% <ø> (-1.01%)	:arrow_down:
setup.py	0.00% <ø> (ø)
rapid_response_xblock/settings.py	100.00% <100.00%> (ø)
tests/test_aside.py	100.00% <100.00%> (ø)
tests/test_events.py	100.00% <100.00%> (ø)
tests/utils.py	92.42% <100.00%> (ø)

[noisecapella]

Code looks good. Reviewing functionality will take a while since I need to update my local edx instance