[deps] Added support for django 3.2 and 4.0 alpha #255

[codesankalp]

Fixed all the tests and issues related to django 3.2 and 4.0 alpha.

Closes openwisp#255

Changes

#Please explain changes made in your PR#

Screenshots

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Checklist

• I have read the contributing guidelines.
• I have manually tested the proposed changes.
• I have written new test cases to avoid regressions. (if necessary)
• I have updated the documentation. (e.g. README.rst)
• I have added [change!] to commit title to indicate a backward incompatible change. (if required)
• I have checked the links added / modified in the documentation.

#Closes #(issue-number)#