gh-127065: Make methodcaller thread-safe and re-entrant (GH-127746)

The function `operator.methodcaller` was not thread-safe since the additional
of the vectorcall method in gh-89013. In the free threading build the issue
is easy to trigger, for the normal build harder.

This makes the `methodcaller` safe by:

* Replacing the lazy initialization with initialization in the constructor.
* Using a stack allocated space for the vectorcall arguments and falling back
  to `tp_call` for calls with more than 8 arguments.