python3: upgrade to 3.7.2

python3: upgrade to 3.7.2

I took the same approach as the recent perl upgrade: write recipe from scratch,
taking the pieces from the old recipe only when they were proven to be necessary.

The pgo, manifest and ptest features are all preserved.

New features:

- native and target recipes are now unified into one recipe

- check_build_completeness.py runs right after do_compile() and verifies that
all optional modules have been built (a notorious source of regressions)

- a new approach to sysconfig.py and distutils/sysconfig.py returning values
appropriate for native or target builds: we copy the configuration file to a
separate folder, add that folder to sys.path (through environment variable
that differs between native and target builds), and point python to the file
through another environment variable.

There were a few other patches where it was difficult to decide if the patch
is still relevant, and how to test that it works correctly; please add those
as-needed by testing the new python.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>