Stop requiring pytest-subtests for python>3.9, it has been incorporated into pytest

[StanFromIreland]

No description provided.

[StanFromIreland]

The pytest versions with incorporated pytest-subtest do not support versions later than 3.10, so we have to keep pytest-subtest for 3.8 and 3.9.

[mtelka]

Maybe something like this should be added too:

pytest >= 9; python_version >= 3.10

Just to make sure you really do have the subtests plugin on Python >= 3.10.