Use mysqlclient under Python 3

edx-platform currently uses the MySQL-python package as its MySQL driver, but this doesn't even install under Python 3. Its fork mysqlclient works under both Python 2 and 3, but we don't have time right now to test switching to it on edx.org. Instead, we'll update the requirements files to use a different package depending on which version of Python is in use:

• Add an environment marker to the MySQL-python dependency in requirements/edx/base.in that restricts its installation to Python 2.7. There are examples of this in requirements/constraints.txt.

• Add a dependency in the same file on mysqlclient, but with a different environment marker which restricts installation to Python versions 3 or greater.

• Launch a development shell via make shell (if you have devstack installed, you can use make lms-shell from there instead).

• Run make upgrade to update the requirements files to reflect the changes.

• Make sure the changes look correct (a few other packages may be upgraded in the process, this is usually fine) and submit a pull request.

• If tests fail, try to diagnose why and either fix them or describe the problem and ask for help.