increment supported Python versions by 1. Minimum version now 3.9

[mperrin]

Implements #535. Drops support for Python 3.8. Astropy is dropping python 3.8 support as of this month so we should follow suite.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2a0c745) 74.78% compared to head (b867b0d) 74.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #550 +/- ## ======================================== Coverage 74.78% 74.78% ======================================== Files 18 18 Lines 6499 6499 ======================================== Hits 4860 4860 Misses 1639 1639 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[mperrin]

Not working as intended. For instance the minimum versions test is inconsistent about numpy version:

py39-legacy-test installdeps: pytest, numpy==1.19.**, astropy==4.0. py39-legacy-test inst: /home/runner/work/poppy/poppy/.tox/.tmp/package/1/poppy-1.0.4.dev19+g14082e4.zip py39-legacy-test installed: astropy==4.0.6,attrs==22.2.0,certifi @ file:///croot/certifi_1671487769961/work/certifi,contourpy==1.0.7,cycler==0.11.0,exceptiongroup==1.1.0,fonttools==4.38.0,importlib-resources==5.12.0,iniconfig==2.0.0,kiwisolver==1.4.4,matplotlib==3.7.0,numpy==1.24.2**

Meanwhile the "latest" version can't build scipy due to not finding openBLAS:

../../scipy/meson.build:130:0: ERROR: Dependency "OpenBLAS" not found, tried pkgconfig and cmake

This clearly needs a bit more massaging...

[BradleySappington]

Taking over with #551