Avoid errors in cases where ``traceback`` is None
Refactor astropy.nddata to not need 2to3
Refactor astropy.stats to use six instead of 2to3
Refactor astropy.time to use six instead of 2to3
Prevent two lists from being created with Python 2
Merge pull request #1518 from astrofrog/fix-exception-handling

Avoid errors in cases where ``traceback`` is None
Update developer docs for warnings
Moved content of utils.custom_warnings into utils.exceptions
Fixes to documentation
Fix tests
Removed debugging print statement
Merge pull request #1517 from astrofrog/minor-warnings-fix

Minor warnings fix
Removed more debug print statements
Merge pull request #1526 from astrofrog/nddata-stats-time-six

Refactor astropy.nddata, astropy.stats, and astropy.time to use six instead of 2to3
Write Unicode guidelines.  Add UNICODE_OUTPUT config option.  Add test
helper to test against guidelines.  Modify astropy.units.Unit to follow
those guidelines, and fix a bug discovered when doing that.
Test roundtrip of __repr__ as well.
Add documentation for repr_namespace
Add changelog entry
Do all roundtrip testing or none.
Fixup arguments to assert_follows_unicode_guidelines