De-instrumenting code objects modifies the thread local bytecode for all threads as such, holding the critical section on the code object is not sufficient and leads to data races. Now, the de-instrumentation is now performed under a stop the world pause as such no thread races with executing the thread local bytecode while it is being de-instrumented.

_testclinic.c mocks out PY_VERSION_HEX to 3.8 before including
_testclinic_depr.c.h to avoid the errors the preprocessor would
otherwise throw due to the deprecation feature it is testing.

Also partially revert 74e2acd:
this restores Modules/_testclinic.c to match the same file in the 3.14
branch.