BinaryField doesn't work on MySQL and Python 3

[Aymeric Augustin]

A unofficial port of MySQLdb appears to be the only solution to connect to MySQL from Python 3 at this time, see [e81e319f].

Unfortunately, its handling of binary data is very buggy. For instance, this code is almost certainly wrong:

def Binary(x):
    return str(x)

There are other problems, like attempting to decode all bytestrings as UTF-8.

With no short-term solution in sight, tests involving BinaryField were marked as expected failures on MySQL and Python 3 in [7476d96].

This commit should be reverted once a production-grade, Python 3-compatible database adapter exists for MySQL.