bpo-32604: Swap threads only if the interpreter is different. (gh-5778)

bpo-32604: Swap threads only if the interpreter is different. (gh-5778)

The CPython runtime assumes that there is a one-to-one relationship (for a given interpreter) between PyThreadState and OS threads. Sending and receiving on a channel in the same interpreter was causing crashes because of this (specifically due to a check in PyThreadState_Swap()). The solution is to not switch threads if the interpreter is the same.