Binder: release the GIL after initialisation (SPEC C-4) #162

SPEC C-4: releasing the GIL after initialisation is not enabled, and the only direct test asserts 'does not crash'. pythonx-concurrent S9.5 needs a Python call that waits for Kotlin not to hold the GIL across the wait; the GIL build needs it for any Kotlin thread to run Python while another waits.
Completion: tests of GIL behaviour beyond not crashing: a second Kotlin thread runs Python while the first waits in Kotlin, on desktop, Android and iOS.