The contents of this attribute are an implementation detail, as
documented for #9442, so we should not parse it, to support non-CPython
VMs in the future.

Unfortunately, one use comes directly from PEP 345, so an edit will have
to be agreed before fixing the code (see comment in d2.markers).

Other remaining uses are found in d2.compiler and could be replaced by
the platform module (which also parses sys.version, but then it wouldn?t
be my fault :)

This should help avoiding issues like #13170.

Because of implicit relative imports, _backport.tarfile already imported
shutil from the backports instead of the stdlib; I have inlined the only
function it needed to remove the circular dependency between these
modules.

I think I?ve got all of them (I?ll clean up compat after); if someone
knows a lint tool that can detect missing super calls, please let me
know.

Backports:
- sysconfig is now always imported from our backports
- when hashlib is not found, our backport is used instead of the md5
  module (debatable; we could just drop hashlib)

Version-dependent features:
- PEP 370 features are only enabled for 2.6+
- the check for sys.dont_write_bytecode was fixed to use getattr
  with a default value instead of hasattr

Idioms/syntax:
- octal literals lost their extra 0
- misused try/except blocks have been changed back to try/finally
  (it?s legal in 2.4 too, it?s only try/except/finally that isn?t)
- exception catching uses the regular 2.x idiom instead of sys.exc_info
- file objects are closed within finally blocks (this causes much
  whitespace changes but actually makes diff with packaging easier)

Renamed modules:
- some missed renamings (_thread, Queue, isAlive, urllib.urlsplit, etc.)
  were fixed

Other:
- a few false positive replacements of ?packaging? by ?distutils2? in
  comments or docstrings were reverted
- util.is_packaging regained its name
- assorted whitespace/comment/import changes to match packaging

The print statement fixes that were in my branch override the print(u'')
calls added by Alexis; the import at function scope were removed for the
usual reasons; d2._backport.hashlib is used if hashlib is not available
instead of md5.