[v1.x] Add Python 3.14 trove classifier

[maxisbey]

Adds the Programming Language :: Python :: 3.14 trove classifier on the v1.x branch.

Motivation and Context

The README's supported-Python-versions badge (img.shields.io/pypi/pyversions/mcp.svg) renders the classifiers of the latest PyPI release, which currently stop at 3.13 — so the badge says 3.10–3.13 even though the SDK installs and works on 3.14. References #2537, where the direction was to add the classifier on v1; the badge updates once the next v1.x release is published with this classifier.

This mirrors the approach used when 3.11–3.13 were added (9bdb8a2). A follow-up PR adds 3.14 to the v1.x CI test matrix so the advertised support is exercised: #2769

How Has This Been Tested?

• Built the package and verified the wheel METADATA carries Classifier: Programming Language :: Python :: 3.14.
• Full v1.x test suite passes locally on 3.14 with 100% coverage (both lowest-direct and highest resolutions) — see the follow-up PR for the matrix change.
• Classifiers aren't resolution inputs, so uv lock --check passes without a lockfile change.

Breaking Changes

None.

Types of changes

• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The badge has shown 3.10–3.13 since 3.14.0 was released (2025-10). Once this merges, cutting a release (e.g. v1.27.3) makes it the latest version on PyPI and the badge updates automatically — no README edits needed on either branch.

_{AI Disclaimer}

[maxisbey]

Folded into #2769 so the classifier and the CI that backs it land together.