restore `mypy` checks on `debian:testing`

[francesco-ballarin]

revert 394020f when debian:testing migrates to PETSc 3.25

[francesco-ballarin]

There are still a few differences: restoring mypy on debian:testing (even after migration to PETSc 3.25) still gives

rbnicsx/_backends/tensors_array.py:197: error: Unused "type: ignore" comment 
[unused-ignore]
            first_output.zeroEntries()  # type: ignore[attr-defined]
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rbnicsx/_backends/tensors_array.py:199: error: Unused "type: ignore" comment 
[unused-ignore]
                first_output.axpy(arg_it, array_it)  # type: ignore[attr-d...
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...
rbnicsx/_backends/tensors_array.py:201: error: Unused "type: ignore" comment 
[unused-ignore]
                first_output.ghostUpdate(  # type: ignore[attr-defined]
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rbnicsx/_backends/tensors_array.py:216: error: Unused "type: ignore" comment 
[unused-ignore]
                first_output_dot_last_arg = first_output.createVecLeft()  ...
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...
rbnicsx/_backends/tensors_array.py:217: error: Unused "type: ignore" comment 
[unused-ignore]
                first_output.mult(args[-1], first_output_dot_last_arg)  # ...
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...
rbnicsx/_backends/tensors_array.py:220: error: Unused
"type: ignore[call-overload]" comment  [unused-ignore]
                return first_output.dot(args[-1])  # type: ignore[no-any-r...
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...

We will keep skipping checks on debian:testing.