Drop support for Python 3.9

[jernejfrank]

Came up during 0.43.0 RC1 release vote:

In a clean Python 3.9 container, installing the RC1 wheel with the [cli] extra
succeeds, but both burr --help and import burr.cli.__main__ fail with:

TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

The failure comes from dict | None in burr/cli/main.py, which requires
Python 3.10+. The release-validation workflow explicitly skips Python 3.9 because of this known incompatibility.

Python 3.9 is EOL so I suggest we just drop the official support for this.

[pjfanning]

It is unusual to release when there are -1s. The norm is to at least try to persuade the voter to change their vote and often new RCs are needed. It is within the rules to proceed with the release but is there any way that you could prominently document the Python 3.9 issue in the release notes and elsewhere?

[jernejfrank]

It is unusual to release when there are -1s. The norm is to at least try to persuade the voter to change their vote and often new RCs are needed. It is within the rules to proceed with the release but is there any way that you could prominently document the Python 3.9 issue in the release notes and elsewhere?

Thanks for bringing this to my attention. I consulted these two docs that both indicated a -1 is not blocking unless it is a veto vote:

• https://incubator.apache.org/policy/incubation.html#releases
• https://www.apache.org/foundation/voting.html#votes-on-package-releases

I will document the problem appropriately in the release notes and here in the repo.

[jernejfrank]

@pjfanning I documented the failure mode in the repo docs: f52464a

Will also add it to the release notes both on Apache mailing list and GH. I will write the release notes when #902 is merged.