LoadLibrary, GetProcAddress, FreeLibrary and GetModuleHandle acquire the system loader lock. Calling these while holding the GIL will cause a deadlock on the rare occasion that another thread is detaching and needs to destroy its thread state at the same time.

Based on patch by Victor Stinner.

Add private C API function _PyUnicode_AsUnicode() which is similar to
PyUnicode_AsUnicode(), but checks for null characters.

Known limitations of the current implementation:

- documentation changes are incomplete
- there's a reference leak I haven't tracked down yet

The leak is most visible by running:

  ./python -m test -R3:3 test_importlib

However, you can also see it by running:

  ./python -X showrefcount

Importing the array or _testmultiphase modules, and
then deleting them from both sys.modules and the local
namespace shows significant increases in the total
number of active references each cycle. By contrast,
with _testcapi (which continues to use single-phase
initialisation) the global refcounts stabilise after
a couple of cycles.