Adapt to availability of `petsc4py` stubs

[schnellerhase]

Upstream stub support for petsc4py has been added, ref. With the next release v3.25.0 we will therefore have access to stubs rendering the related type: ignore's superfluous.

(slepc4py support pending)

Would be good to check these before the release happens.

[jhale]

Ah this is great news.

Our stub workflow also needs looking at (checking that we are using auto-generated stubs, and that the output of those stubs is good).