Add pytest-mrt to Plugins section

[croc100]

I've been bitten a few times by migrations that applied cleanly but failed on rollback — sometimes silently, sometimes with data loss. Built pytest-mrt to catch these before they hit production.

It does two things: static analysis on migration files (44 patterns, no database needed), and dynamic verification that actually runs the up/down/up cycle against a real database and checks that schema and data survive the round trip. Works with both Alembic and Django.

• PyPI: https://pypi.org/project/pytest-mrt/
• MIT, actively maintained