fix(ci): switch Dependabot Python ecosystem from uv to pip

[dpgaspar]

SUMMARY

The uv package ecosystem support in Dependabot is still in beta and has been unreliable — Dependabot alerts for Python dependencies hang indefinitely when attempting to open PRs (e.g. https://github.com/apache/superset/security/dependabot/1150).

This switches the Python dependency section from uv to pip, which natively understands requirements/*.txt files. The uv ecosystem primarily targets projects with a uv.lock file, which this project doesn't use (we use uv pip compile to generate .txt files).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

• After merge, verify that Dependabot can successfully create PRs for Python dependency updates
• Check the Dependabot security tab for any previously stuck Python alerts

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

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[rusackas]

LGTM

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