Docs: improve covariance generic typing examples

[dantetemplar]

---

📚 Documentation preview 📚: https://cpython-previews--130302.org.readthedocs.build/

[ghost]

All commit authors signed the Contributor License Agreement.

[dantetemplar]

skip issue

[dantetemplar]

The original documentation did not include a declaration for BasicUser, so I added one along with some additional examples

[JelleZijlstra]

I don't think we need this many additional examples. Let's try to keep the examples short and focused. It may be enough to just add a class BasicUser(User): ....

[dantetemplar]

I don't think we need this many additional examples. Let's try to keep the examples short and focused. It may be enough to just add a class BasicUser(User): ....

Let me argue, then this section does not give the reader information about how typing with inheritance will work in this case. If it’s ok, then fine, I’ll remove the unnecessary stuff.

[JelleZijlstra]

This now demonstrates the exact same thing three times. I think we only need one. We can keep one of the three subclasses in this test, and move the remaining two class definitions to the next test.