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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/cta-observatory/pyirf/pull/243?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cta-observatory) | Coverage Δ | | |---|---|---| | [pyirf/spectral.py](https://app.codecov.io/gh/cta-observatory/pyirf/pull/243?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cta-observatory#diff-cHlpcmYvc3BlY3RyYWwucHk=) | `76.92% <66.66%> (-0.39%)` | :arrow_down: |

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