Django, up through 1.11, made an assumption that mysqlclient's

database connection class had bytes in a mapping of internal data type

converters. This assumption worked in Python 2.7, since bytes and

str were the same thing, but it broke on Python 3. The mysqlclient

developers temporarily added compatibility for this, but have recently

removed that as of mysqlclient 2.1, since Django 2.0+ doesn't exhibit

this problem.
This change adds a patch to work around that problem. It pre-populates

the needed bytes mapping in the conversion map that Django sets up and

passes to mysqlclient. Django's improper logic will still kick in, but

it will no longer fail on newer versions of mysqlclient without that

workaround.

Tested MySQL support on Python 3 on Django 1.6 through 2.0, with

mysqlclient 1.4.6, 2.0, and 2.1.