python 3 uses only the 'str' type, while python 2 also uses 'unicode'.
This patch adds a type_is_str() function to common.py - depending on the
python version, it checks for both. This helper function is used to keep
the complexity outside of the rule classes.
The rule classes get adjusted to use type_is_str() instead of checking
for type(x) == str, which means they support both python versions.
As pointed out by Tyler, there are also some type(...) == str checks in
aare.py and rule/__init__.py which should get the same change.
Finally, add test-common.py with some tests for type_is_str().
References: https://bugs.launchpad.net/apparmor/+bug/1513880
Acked-by: Tyler Hicks <tyhicks@canonical.com> for trunk and 2.10
Note: 2.10 doesn't contain SignalRule, therefore it doesn't get that
part of the patch.
