fix: mark awkward as freedthread safe with pybind11

[lgray]

Fixes: #3395

Need to create some tests that demonstrate this is, in fact, the case.

[lgray]

My understanding of the C component of awkward array is that it is already reentrant and carries no state.
So I think this is safe to do without adding tests aside from the usual tests in python 3.13t (which is soon to be available through actions).

I am thinking about the python side of awkward and there are not many situations where you'd be manipulating array metadata in a reentrant way, no structure we can make is large enough to need it. Typically multithreaded work on awkward arrays is going to be at the numpy level.

The only place where there might be some issues is with typetracers, but the this is a fairly ill-defined worry.

My gut feeling is that we don't really need to do too much to merge this.