Add pandas 2+3 CI matrix testing

[MaxGhenis]

Summary

• Adds pandas-version: ["2", "3"] to the CI test matrix
• After installing dependencies, forces the specific pandas major version
• Ensures microdf remains compatible with both pandas 2.x and 3.x
• Follows up on the recent pandas 3.0 compatibility fixes (Add pandas 3.0 compatibility tests #270, Fix pandas 3.0 compatibility issues with MicroSeries method access #276)

Test plan

• CI passes with pandas 2 matrix
• CI passes with pandas 3 matrix

🤖 Generated with Claude Code

[MaxGhenis]

Closing as stale (idle since 2026-02-03; the PR matrix already runs Python 3.9-3.13 with unpinned pandas, which resolves pandas 2 on 3.9/3.10 and pandas 3 on 3.11+, and #337 withdraws wider pandas coverage from the roadmap). Max's PR sweep 2026-09-25. Reopen if still wanted.