Use mysqlclient instead of mysql-python

This PR swaps out mysql-python with mysqlclient in the requirements, the latter is available for python3 while the former is not.

requirements.txt

@@ -22,7 +22,7 @@ ligo-lvalert-overseer==0.1.3
 lxml==4.2.0
 matplotlib==2.0.0
 mock==2.0.0
-mysql-python==1.2.5
+mysqlclient==1.4.2
 packaging==17.1
 phonenumbers==8.8.11
 python-ldap==3.1.0