tests: Add pytest to test if there are any issues

This commit adds basic structure to add tests in mailman-web, which so far has been missing. Currently, we only look for basic doc and style level changes.

Going forward, we want to make mailman-web as the "integration" testing for Mailman's Web components to ensure that they all work together even if their individual test suites work.

We will need to test with a growing combination of package releases for django and mailman versions, so we'll need to establish a testing matrix against which we will need to run the tests.