fix: minor updates to mypy page

[henryiii]

Adding some help for runtime annotation usage & some updates for 3.7+ min.

[kratsg]

I think this somewhat clarifies things. It just was not obvious to me that:

• static code analysis will rewrite anything with from __future__ import annotations for the PEP585 "upgrade" for you (regardless of what python version you end up using)
• code that needs to evaluate typehints at runtime (looking at typer) will need to work on older python where this runtime evaluation will fail (after the upgrade), while using click with the same typehints + the PEP585 applied will not crash (because those typehints are not evaluated explicitly by anything)