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.
Pieter Eendebak