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.

PyModule_GetDef() returns an error.

in Python/dynload_shlibs.c.

This should fix the remaining importlib test failure on Windows.
Support in AIX and HP-UX will be in a separate checkin.

importlib._bootstrap is now frozen into Python/importlib.h and stored
as _frozen_importlib in sys.modules. Py_Initialize() loads the frozen
code along with sys and imp and then uses _frozen_importlib._install()
to set builtins.__import__() w/ _frozen_importlib.__import__().

The name must be encodable to ASCII because dynamic module must have a function
called "PyInit_NAME", they are written in C, and the C language doesn't accept
non-ASCII identifiers.

on Windows.