Support django 4.2, 5.0

[getogrand]

Hi! I really appreciated this great middleware.

Fortunately, this library seems to work with the latest version of Django and Python. This change makes this explicit and adds a test setup for it.

Also, I've added a few changes related to the Github Action workflow.

The GitHub Actions of this repository is not working currently due to the GitHub hosted-runners dropped ubuntu-18.04 (now supports ubuntu-24.04, ubuntu-22.04, ubuntu-20.04). So I've bumped up runners OS to unbuntu-20.04 and dropped Python 2.7 and 3.5 support which is not supported from unbuntu-20.04.