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Most of the import sequence now uses per-module locks rather than the
global import lock, eliminating well-known issues with threads and imports.

Issue #14583: Fix importlib bug when a package's __init__.py would first import one of its modules then raise an error.

This introduces a new function, imp.extension_suffixes(), which is
currently undocumented. That is forthcoming once issue #14657 is
resolved and how to expose file suffixes is decided.

Thanks to Eric Snow for the patch.

finder instead of using some (now non-existent) implicit finder.

 * In debug mode, fill the string data with invalid characters
 * Simplify also reference counting in PyCodec_BackslashReplaceErrors()
   and PyCodec_XMLCharRefReplaceError()

Thanks to Eric Snow for taking an initial stab at the implementation.

Being able to overload a sys.module entry during import of a module was broken
by this changeset.

or __name__ are not set in globals.

Thanks to Stefan Behnel for the bug report.

Thanks to Eric Snow for helping with imp.load_module() (previous
commit) which led to the removal of a bunch of C code.

__loader__.

Since import now sets __loader__ on all modules it creates and
imp.reload() already relied on the attribute for modules that import
didn't create, the only potential compatibility issue is if people
were deleting the attribute on modules and expecting imp.reload() to
continue to work.

rewriting functionality in pure Python.

To start, imp.new_module() has been rewritten in pure Python, put into
importlib (privately) and then publicly exposed in imp.

of sys.modules when possible.

This is being done for two reasons. One is to gain a little bit of
performance by skipping an unnecessary dict lookup in sys.modules. But
the other (and main) reason is to be a little bit more clear in how
things should work from the perspective of import's interactions with
loaders. Otherwise loaders can easily forget to return the module even
though PEP 302 explicitly states they are expected to return the module
they loaded.

found in sys.modules.

__import__('mod', {'__packaging__': 'pkg', level=1) w/o properly (and
thus not segfaulting).

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__().

allocating path buffers on the heap instead of the stack.

This allows generators that are using yield from to be seen by debuggers. It
also kills the f_yieldfrom field on frame objects.

Patch mostly from Mark Shannon with a few tweaks by me.