* Backports ensurepip to the 2.7 branch
* Backports some of the improved documentation to the 2.7 branch.
* Adds a private backport of the 3.x mock library as test._mock_backport
  to enable saner testing of ensurepip.

Key Differences from 3.x:

* Ensurepip does not have any Makefile integration, specifically
  it is not ran by default in the Makefile.
* There is no venv module in 2.7, so downstream distributors can
  completely disable ensurepip, ideally with a message redirecting
  to the correct way to install pip.
* To match the ``python`` command in 2.7, ensurepip will install
  the unversioned ``pip`` command as well.
* No-op and hide --default-pip and add --no-default-pip to restore
  the 3.x behavor on 2.7.

different lowercase are now matched in case-insensitive regular expressions.

platforms.

(issue #814253).

work in lookbehind assertions in regular expressions.

Based on patch by Aivars Kalvāns.

Issue #20160: broken ctypes calling convention on MSVC / 64-bit Windows (large structs). Patch by mattip

Patch by Tim Graham.

#22751: fix test___all__ warning about modified environment in the tests.  Patch by Michael Cetrulo.

When running tests in subprocesses with the regrtest -j option, a bug
in Cocoa Tk can result in a Tcl segfault.  Avoid the problem by forcing
Tk to fully initialize as an OS X gui process before destroying the
Tcl instance and creating another. (Original patch by Serhiy Storchaka)

(issue #19329).  This is needed to apply the patch from issue #17381.