add check that the GIL is not re-enabled for the free-threading python migrations

[beckermr]

@mariusvniekerk suggested that we should add a check that the GIL is not re-enabled for free-threading Python migrations. He suggested using either an environment variable to force the GIL to not be re-enabled or to import the package and turn warnings into errors.

We can add a mini-migrator on top of the python one that adds to the tests.

[beckermr]

cc @zklaus @jaimergp

[beckermr]

https://docs.python.org/3/howto/free-threading-python.html#warning-filters