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).

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