Conditionally load stubs based on specific version of package installed

For django-stubs, we would like to be able to load stubs conditionally, like it's done for typeshed via https://mypy.readthedocs.io/en/latest/common_issues.html#python-version-and-system-platform-checks

See https://github.com/typeddjango/django-stubs/issues/262

I'd like to propose to add a special function is_package_installed for stub files:

if is_package_installed('Django>=3.0.*')
    class Django30Class: ...
else:
    class Django22Class: ...

which will evaluate according to the rules specified in PEP440 https://www.python.org/dev/peps/pep-0440/#version-specifiers

It was discussed in https://github.com/python/typeshed/issues/153, current workaround seems to be (from https://github.com/python/typeshed/issues/153#issuecomment-389198282):

release and install a separate stub package for each version of the library

I believe it's too complex, as one needs to maintain lots of separate branches for specific versions of the supported package. And versioning of the stubs package would then be PACKAGE_VERSION.STUBS_VERSION_INCREMENT?

Start with the linked mypy documentation and PEP 440 version-specifier rules, then read the referenced django-stubs and typeshed discussions. The proposed behavior is a design for conditionally selecting stub declarations based on an installed package version; done requires an agreed mechanism and documented behavior for supported version checks.