Clarified comment on disabled code to reference issue15093.

Add relevant tests for GzipFile, and also for BZ2File and LZMAFile.

Issue #14928: Fix importlib bootstrap issues by using a custom executable (Modules/_freeze_importlib) to build Python/importlib.h.

Issue #15103: remove the NUL character (serving as a Mercurial binary marker) from Python/importlib.h.
Instead the email notification hook uses a configuration option to omit importlib.h diffs.

Fix incorrect test of the condition variable state, spotted by
Richard Oudkerk.  This could cause the internal condition variable
to grow without bounds.

Extract cross-platform condition variable support into a separate file and
provide user-mode non-recursive locks for Windows.

Prevent test_inspect from keeping alive a ton of frames and local variables by way of a global variable keeping a reference to a traceback.
Should fix some buildbot failures.

Issue #14657: The frozen instance of importlib used for bootstrap is now also the module imported as importlib._bootstrap.