tests: try to import modules with Python 3
All of mercurial.* is now using absolute_import. Most of
mercurial.* is able to ast parse with Python 3. The next big
hurdle is being able to import modules using Python 3.
This patch adds testing of hgext.* and mercurial.* module imports
in Python 3. As the new test output shows, most modules can't
import under Python 3. However, many of the failures are due
to a common problem in a highly imported module (e.g. the bytes vs
str issue in node.py).