Upgrade dependecy of typing_extensions to ^4.0.0

Hello,

I would like to know if it’s possible to upgrade the dependency to typing_extensions >= 4.0.0.

As of now we cannot introduce some external dependencies like pydantic in our project because of dependency conflicts with django-test-migrations. I don’t exactly know if some breaking changes have been introduce in v4, however on the project README we can read the following:

Starting with version 4.0.0, typing_extensions uses Semantic Versioning. The major version is incremented for all backwards-incompatible changes, including dropping support for older Python versions. Therefore, it’s safe to depend on typing_extensions like this: typing_extensions >=x.y, <(x+1), where x.y is the first version that includes all features you need.

[sobolevn]

Please, make a PR! 👍

[sobolevn]

Done!