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

[codesankalp]

Changes

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

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 #255

[codesankalp]

I tested locally for python==3.7 and django=4.0a1, it is working fine. But in CI the Django alpha version is not present while installing: https://github.com/openwisp/openwisp-utils/pull/258/checks?check_run_id=3893636475#step:6:121

[nemesifier]

It seems Django 4 is not compatible with python 3.7, so we should avoid running tests with Django 4 and python 3.7:
https://docs.djangoproject.com/en/dev/releases/4.0/

[coveralls]

Coverage remained the same at 98.431% when pulling 4fd3110 on issue-255-django-version-support into 9e182fd on master.

[nemesifier]

this should be 3.2.0 for consisntency.

[codesankalp]

There were bug in 3.2.0, so I thought to use 3.2.8: https://docs.djangoproject.com/en/3.2/releases/3.2.8/

[nemesifier]

@codesankalp there's always bugs in the initial minor versions of a release, the build will install the latest available version anyway. It's a matter of clarify, readability and consistency. By indicating ~=3.2.0 we know the build will use the latest 3.2.x version.

[nemesifier]

Tested and it works, thanks! A couple minor improvements requested below.