Switch from MySQL-python to PyMySQL

For the same reason nodepool switched[1], let's switch jeepyb too.

[1] http://git.openstack.org/cgit/openstack-infra/nodepool/commit/?id=63e5297f7e56bd2f8ca8c16e1f1b032f6c6f5ab8

Change-Id: I3afeaebeb8416388d5866fc3994357935079c97d
diff --git a/requirements.txt b/requirements.txt
index 65d5218..9390472 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,7 +2,7 @@
 
 argparse
 gerritlib>=0.3.0
-MySQL-python
+PyMySQL
 paramiko
 PyGithub
 pyyaml