BLD: fix python version in test images

I discovered the source of the Python 3.9 CI failures (along with general issues with pymc.)

I had accidentally pinned the Python version to the first release (e.g., 3.9.0, 3.10.0, ...), see here. The Python 3.9 environment now builds and the tests pass for me, hopefully they also will in our CI.

changed milestone to %2.4.x

added <10 lines Infrastructure Testing labels

approved this merge request

Thanks for figuring this out, Colm!

approved this merge request

merged

mentioned in issue #728 (closed)

changed milestone to %2.3.0