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Python 3 is now better supported by the tests, and should be nearly
working with these changes. There are also a few changes to try to make
it easier to work on Windows, but this is still not working properly.

When importing this module into other projects, it is important that
we can actually see the imports we've created. It's only a very
simple import line, but very important!

The output on stderr being merged with the stdout was causing problems
when errors were output on that stream. We now have the option to
not write them out there.

The bootstrap-env wasn't working properly because we didn't give a default
for $TMPDIR. We now use /tmp as the default.

Provides callback functions which are supplied with data as it is
received from a subprocess.