BLD: Set minimum Python to 3.9

[matthewfeickert]

• Update the python_requires to Python 3.9, which is the oldest not-EOL Python as of 2025-10-28, and the oldest Python tested against in CI.

$ pixi exec norwegianblue python
┌───────┬────────────┬─────────┬────────────────┬────────────┬────────────┐
│ cycle │  release   │ latest  │ latest release │  support   │    eol     │
├───────┼────────────┼─────────┼────────────────┼────────────┼────────────┤
│ 3.14  │ 2025-10-07 │ 3.14.0  │   2025-10-07   │ 2027-10-01 │ 2030-10-31 │
│ 3.13  │ 2024-10-07 │ 3.13.9  │   2025-10-14   │ 2026-10-01 │ 2029-10-31 │
│ 3.12  │ 2023-10-02 │ 3.12.12 │   2025-10-09   │ 2025-04-02 │ 2028-10-31 │
│ 3.11  │ 2022-10-24 │ 3.11.14 │   2025-10-09   │ 2024-04-01 │ 2027-10-31 │
│ 3.10  │ 2021-10-04 │ 3.10.19 │   2025-10-09   │ 2023-04-05 │ 2026-10-31 │
│ 3.9   │ 2020-10-05 │ 3.9.24  │   2025-10-09   │ 2022-05-17 │ 2025-10-31 │
│ 3.8   │ 2019-10-14 │ 3.8.20  │   2024-09-06   │ 2021-05-03 │ 2024-10-07 │
│ 3.7   │ 2018-06-27 │ 3.7.17  │   2023-06-05   │ 2020-06-27 │ 2023-06-27 │
│ 3.6   │ 2016-12-23 │ 3.6.15  │   2021-09-03   │ 2018-12-24 │ 2021-12-23 │
│ 3.5   │ 2015-09-13 │ 3.5.10  │   2020-09-05   │   False    │ 2020-09-30 │
│ 3.4   │ 2014-03-16 │ 3.4.10  │   2019-03-18   │   False    │ 2019-03-18 │
│ 3.3   │ 2012-09-29 │ 3.3.7   │   2017-09-19   │   False    │ 2017-09-29 │
│ 3.2   │ 2011-02-20 │ 3.2.6   │   2014-10-12   │   False    │ 2016-02-20 │
│ 2.7   │ 2010-07-03 │ 2.7.18  │   2020-04-19   │   False    │ 2020-01-01 │
│ 3.1   │ 2009-06-27 │ 3.1.5   │   2012-04-06   │   False    │ 2012-04-09 │
│ 3.0   │ 2008-12-03 │ 3.0.1   │   2009-02-12   │   False    │ 2009-06-27 │
│ 2.6   │ 2008-10-01 │ 2.6.9   │   2013-10-29   │   False    │ 2013-10-29 │
└───────┴────────────┴─────────┴────────────────┴────────────┴────────────┘

[matthewfeickert]

@tacaswell Not sure how you feel about changing lower bounds by a lot, but might allow for some easier updates in the future to know that old Python support won't be affected.

[tacaswell]

Can you add something to CHANGES.rst?