Revert "Ensure that tmpdir is unicode. Fixes #704."

This reverts commit 85794957.

As we can see on https://github.com/pypa/setuptools/issues/709,
this breaks many things (easy_install C extensions, all py3.5 tests,
run with LANG=C).

So instead of fixing in a hurry all new bugs due to this, I propose to
revert this commit until all downsides of this change have been
investigated.

Related bug: https://github.com/pypa/setuptools/issues/709
Related bug: https://github.com/pypa/setuptools/issues/710
Related bug: https://github.com/pypa/setuptools/issues/712

fix NameError when initial working_set is empty

Fix for #646 : Wrong error caught in RegistryInfo.lookup