Detect incompatibile mypy version and print a warning

[intgr]

Every now and then, users report issues when using the django-stubs mypy plugin with old mypy versions that are not compatible, which may be due to being unaware of the coupling between these packages, or just by mistake. E.g. #1784, #1434.

Mypy's plugin API has been pretty stable recently, but it still happens occasionally that we introduce dependency on new mypy features or APIs that breaks older mypy, e.g. #1703

So to save developer time as well as users' time, we should consider issuing a warning or error message, to make debugging this easier.

Of course there's a balance to be struck here. In my projects, I often upgrade mypy out of sync with django-stubs's official compatible-mypy; mypy's version is sometimes one major version ahead or behind compatible-mypy. And I think we shouldn't prevent that use case.