Optimize pending calls in multithreaded applications. If a thread different
than the main thread schedules a pending call (:c:func:`Py_AddPendingCall`),
the bytecode evaluation loop is no longer interrupted at each bytecode
instruction to check for pending calls which cannot be executed. Only the
main thread can execute pending calls.

Previously, the bytecode evaluation loop was interrupted at each instruction
until the main thread executes pending calls.
