Drop python 3.9

[mraspaud]

This PR drops support for python 3.9, as deemed reasonable in #2741. This superseeds #2741 which was difficult to merge with main. ~Also, we added an experimental build for python 3.13.~

• [x] Closes #2741

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.09%. Comparing base (5634073) to head (f9dccad). Report is 77 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2936 +/- ## ======================================== Coverage 96.08% 96.09% ======================================== Files 375 377 +2 Lines 54615 55075 +460 ======================================== + Hits 52478 52925 +447 - Misses 2137 2150 +13 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2936/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2936/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `3.95% <ø> (-0.04%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2936/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.19% <ø> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[mraspaud]

hmm, looks like the 3.13 experimental was a bad idea...

[coveralls]

Pull Request Test Coverage Report for Build 11384578870

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.01%) to 96.202%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/readers/file_handlers.py	1	97.4%
satpy/readers/sgli_l1b.py	1	95.17%
satpy/readers/xmlformat.py	1	88.79%
satpy/tests/reader_tests/test_ahi_hsd.py	1	99.34%
satpy/readers/pmw_channels_definitions.py	2	96.41%
<!--	Total:	6		-->

Totals
Change from base Build 11381112172:	-0.01%
Covered Lines:	53169
Relevant Lines:	55268

---

💛 - Coveralls

[djhoese]

Were all the FCI/decomp requirements/requests related to dropping Python 3.9 resolved?

[mraspaud]

Yes I think so

[mraspaud]

The last comment on #2741 confirms it.