Summary:
Automated migration to enable Pyrefly type checking for `fbcode/mcrouter`.

- Added `python.set_pyrefly(True)` to PACKAGE file
- Suppressed pre-existing type errors

Pyrefly is Meta's next-generation Python type checker, replacing Pyre.

If you encounter issues, you can revert the PACKAGE change by removing
the `python.set_pyrefly(True)` line.
#pyreupgrade

Differential Revision: D103425534

fbshipit-source-id: dbea23f732c5b6d3a9e8e2f0de0c75a8803b6ad4