Package compat

SCons compatibility package for old Python versions

This subpackage holds modules that provide backwards-compatible
implementations of various things that we'd like to use in SCons but which
only show up in later versions of Python than the early, old version(s)
we still support.

Other code will not generally reference things in this package through
the SCons.compat namespace.  The modules included here add things to
the builtins namespace or the global module list so that the rest
of our code can use the objects and names imported here regardless of
Python version.

Simply enough, things that go in the builtins name space come from
our _scons_builtins module.

The rest of the things here will be in individual compatibility modules
that are either: 1) suitably modified copies of the future modules that
we want to use; or 2) backwards compatible re-implementations of the
specific portions of a future module's API that we want to use.

GENERAL WARNINGS:  Implementations of functions in the SCons.compat
modules are *NOT* guaranteed to be fully compliant with these functions in
later versions of Python.  We are only concerned with adding functionality
that we actually use in SCons, so be wary if you lift this code for
other uses.  (That said, making these more nearly the same as later,
official versions is still a desirable goal, we just don't need to be
obsessive about it.)

We name the compatibility modules with an initial '_scons_' (for example,
_scons_subprocess.py is our compatibility module for subprocess) so
that we can still try to import the real module name and fall back to
our compatibility module if we get an ImportError.  The import_as()
function defined below loads the module as the "real" name (without the
'_scons'), after which all of the "import {module}" statements in the
rest of our code will find our pre-loaded compatibility module.

Attempts to import the old module and load it under the new name.
Used for purely cosmetic name changes in Python 3.x.