Disable mypy on petsc4py 3.20 types

[garth-wells]

Somethig must have changed in PETSc.

[francesco-ballarin]

For reference, this is the upstream issue that tracks this https://gitlab.com/petsc/petsc/-/issues/1367

It is a nightmare to add # type: ignore at every single PETSc line. In my CI I simply remove the py.typed file from petsc4py see
https://github.com/RBniCS/RBniCSx/blob/main/.github/workflows/ci.yml#L38
since the above issue establishes that petsc4py is not really typed. I do realize that it may not be the ideal solution, since each developer would have to remember to do that in their own petsc4py installation.