pythonmodule: Release the GIL in tb_poll_event / tb_peek_event.

The peek/poll methods of Termbox both acquire the GIL, so while
these functions block, no other thread can do anything.

Solution: Declare both methods as "nogil".
pythonmodule: Add locking to peek_event/poll_event.

After we have removed the GIL for these two methods, we should
ensure that no two threads try to peek/poll at the same time.

I'm not sure if this is really needed, but it should not hurt either.
termboxmodule: Fix locking

Fix compiler error.
Merge branch 'upstream'

Conflicts:
	src/termboxmodule.pyx