Update regression tests configuration

[nden]

Changes in this PR:

• Remove testing with Python 3.8 (Py 3.8 is not supported any more)
• Enable testing with Python 3.9, 3.10 and 3.11
• Start testing with development versions of dependencies - astropy and numpy
• Fix the RTD build (systems_packages was deprecated)

Regression test run is here https://plwishmaster.stsci.edu:8081/job/RT/job/Calcos-Developers-Pull-Requests/1/

All failures are in the devdeps job and indicate problems with the upcoming numpy 2.0 release.

[codecov[bot]]

Codecov Report

Patch has no changes to coverable lines.

:loudspeaker: Thoughts on this report? Let us know!.

[stscirij]

The numpy exceptions on the development branch come from the astropy.units library

[nden]

Another run is here: https://plwishmaster.stsci.edu:8081/job/RT/job/Calcos-Developers-Pull-Requests/2/

[nden]

@stscirij What do you think about this PR?