Clean up imports in hostingsvcs and scmtools tests

This cleans up the now-obsolete md5-vs-hashlib fallbacks, as well as imports
much more specific items (in most of scmtools/tests.py's imports, we only cared
about a single symbol).

Testing done: Ran unit tests.
Reviewed at http://reviews.reviewboard.org/r/4294/
Fix Clear Case popen problems on Python 2.7.x.

On Windows 7, using Python 2.7.x, Clear Case would pop up a console
window every time popen was invoked. We now change the shell= parameter
depending on whether we're running Windows 7 and Python 2.7.
Merge branch 'release-1.7.x'