Python3 support

[sdwfrost]

It would be great to get Pylogeny working with Python3:

• Remove dependency on mysqldb-python
  • Perhaps use mysqlclient as a drop-in replacement that works for both Python2 and Python3
• Tweak the Python wrapper in fitch.cpp (involving Python2/3 differences in how strings are treated and int vs. long, described here)
• Some minor syntax changes

Unfortunately, the p4 dependency also is incompatible with Python3, which requires some additional work.