Update python to >= 3.9

[RuneDominik]

As mentioned by @maxnoe in #242 we could drop python 3.8 support in accordance with NEP 29.

Below: comments from #242

@maxnoe:

As per NEP-29, we can also drop 3.8 now:

On Apr 14, 2023 drop support for Python 3.8 (initially released on Oct 14, 2019)

@RuneDominik:

If we want this I'll make this a new PR, arguably we could wait for ctapipe to make this move as pyirf likely will become a dependency there sometime and we may want to aim for consistency throughout cta-observatory packages. However, even them requiring python >= 3.8 and us python >= 3.9 is ofc. solvable.

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.01 ⚠️

Comparison is base (05a5274) 94.33% compared to head (46349ad) 94.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
- Coverage   94.33%   94.33%   -0.01%     
==========================================
  Files          58       58              
  Lines        2738     2736       -2     
==========================================
- Hits         2583     2581       -2     
  Misses        155      155              

Impacted Files	Coverage Δ
pyirf/spectral.py	76.92% <66.66%> (-0.39%)	⬇️

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