Fixes issue 61 compatibility with hg 5.4 and 5.5
Fixes and adaptations for compatibility with hg 5.4 and hg 5.5. The contents is mainly compatibility with Python 3 and some tweaking of the tests to account for exit codes that have changed.
Of note:
- The introduction of a new oracle class for determining whether an error message denotes a conflict;
- The inclusive range of supported versions extends from 3.7 to 5.5.