docs: align contributor Python version docs with package metadata

[immanuwell]

Which issue(s) does this change fix?

N/A

Why is this change necessary?

The contributor docs still point people at Python 3.8 and 3.9, but the package metadata requires 3.10+. So the old setup path is stale and folks hit a wall pretty fast.

How does it address the issue?

Updates CONTRIBUTING.md and DEVELOPMENT_GUIDE.md to match pyproject.toml and the current CI matrix. Also drops a couple old setup.py and AppVeyor refs.

Repro:

1. Follow the old guide and use Python 3.9.
2. Run uv venv --python 3.9 /tmp/samcli-doc-repro/venv && uv pip install --python /tmp/samcli-doc-repro/venv/bin/python -e .
3. See the resolver fail, eg: Because the current Python version (3.9.25) does not satisfy Python>=3.10

What side effects does this change have?

Docs only. No runtime change, no test behavior change

[aws-sam-cli-bot]

Code Review Results

Reviewed: 58da8ea..1c3f021
Files: 2
Comments: 1

[aws-sam-cli-bot]

[GENERAL] The rename of the example virtualenv from samcli38 to samcli310 is incomplete. Line 145 in the "Install development version of SAM Transformer" section still reads:

pyenv activate samcli38  # if you chose to use pyenv to setup the virtual environment

After this PR, the prerequisites section instructs contributors to create a virtual environment named samcli310, so a reader who follows the new guide top-to-bottom will be told here to activate samcli38, which they were never instructed to create. Since this PR's stated goal is to align contributor docs with the current Python version, this leftover reference should be updated to samcli310 (and the surrounding sentence checked for any other stale Python 3.8/3.9 wording) for consistency.

[immanuwell]

Fixed. I updated the leftover pyenv activate samcli38 example to samcli310 and re-checked the surrounding contributor-doc references for other stale Python 3.8/3.9 env names.