before this commit: fetch_tests and release_yum_tests
allow *any* dist tag, but the tag must not be empty or
else the glob fails on double-dot in path name.
after this commit: allow the tag to be any *or* empty.
python3 does not allow relative imports.
test/unit/__init.py__ and test/functional/__init.py__ already exist,
which means we can use absolute import on all versions of python
without resorting to http://legacy.python.org/dev/peps/pep-0328/
Add modules compat lib to support both python 2 and 3, and
add unit test to detect regressions.
Tito uses two modules that have changed significantly between
python 2 and 3:
* commands module merged into subprocess module
* ConfigParser module renamed as configparser
