math/py-pandas: Update to 0.25.2, add math/py-pandas024 for py27 flavor

[john_saltant.com]

Summary

Changelog

• math/py-pandas
  • Update math/py-pandas to 0.25.2
  • Chase minimum versions of (soft) dependencies
  • Drop support for Python 2
• math/py-pandas024
  • New port
  • repocopy from r511602, plus some minor changes incl. CONFLICTS, pkg-descr, PORTSCOUT [note]
• Mk/Uses/python.mk
  • Add PY_PANDAS macro to automatically depend on best available version
• py27-compatible ports depending on pandas
  • Use PY_PANDAS macro
  • Bump PORTREVISION

[note] I couldn't figure out how to get the repocopy to show up in the svn diff, so this reflects a simple add that magically appears.

Test Plan

portlint on math/py-pandas, and math/py-pandas024
poudriere bulk ports touched by this patch with "all" meta-flavor

[john_saltant.com]

Add missed math/Makefile, and move macro threshold from 3300 to 3500.

[john_saltant.com]

Bye bye.