Manage the CI Python interpreter with uv

[tpvasconcelos]

Description

Let uv fully manage the Python interpreter in CI, as part of the uv adoption tracked in #219. The setup-python composite action now uses astral-sh/setup-uv's python-version and activate-environment inputs and drops actions/setup-python entirely:

• The requested Python version is installed as a uv-managed interpreter (python-build-standalone builds) instead of GitHub's toolcache CPython.
• setup-uv creates and activates a virtual environment used by all subsequent steps, so the requirements install step targets it directly (dropping --system). Tox then uses the venv's interpreter as its host Python, so the test matrix versions are honored exactly as before.

Note: this change previously ran on main as eed5995 (accidental direct push, since reverted in a70141e so it could be reviewed here). All workflows passed on that commit, including the full 3.10-3.14 test matrix, the release workflow's dist build, and the docs build - so the change is already CI-validated end-to-end.

Related issues

Related to #219.

PR check list

• Read the contributing guidelines.
• Provided the relevant details in the PR's description.
• Referenced relevant issues in the PR's description.
• Added tests for all my changes. (N/A - CI config only; validated by the CI runs on this PR)
• Updated the docs for relevant changes. (N/A)
• Changes are documented in docs/reference/changelog.md.
• The CI check are all passing, or I'm working on fixing them!
• I have reviewed my own code! 🤠

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📚 Documentation preview 📚: https://ridgeplot--393.org.readthedocs.build/en/393/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a70141e) to head (c2c4ca7).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #393   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           19        19           
  Lines          674       674           
  Branches        88        88           
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  Hits           674       674           

Flag	Coverage Δ
combined-src	100.00% <ø> (ø)

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