It looks like method _ModuleLock.acquire() was re-entered in the same thread.

* Enter acquire() first time, set _blocking_on[tid].
* Trigger some callback (profiler, debugger, tracemalloc) which uses import and calls acquire() again from the same thread.
* Enter acquire() second time, set _blocking_on[tid].
* Delete _blocking_on[tid] and leave the second acquire().
* Try to leave the first acquire() and delete _blocking_on[tid] again.