upgrade to django 3.2

[rlskoeser]

retest notes

• test admin add work to a collection to confirm it works and does not result in a 500 error

dev notes

• upgrade django 3.2
• upgrade wagtail 2.15
• update staging server to python 3.6
• update staging deploy to python 3.6
• remove grappelli

after testing:

• trying to add a work to a collection on the backend results in a 500 error

[mnaydan]

Looks good! I tested adding work(s) to one and multiple collections through the bulk add feature and through the individual edit form. All passed!