Docs: improve covariance generic typing examples

[dantetemplar]

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

[cpython-cla-bot]

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[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.