Depend on mysqlclient instead of MySQL-Python.

The latter is not supported, hasn't been touched in years and isn't
packaged in any OS anymore (the only reason I was using thus far).

setup.py

@@ -26,7 +26,7 @@ setup(
     ],
     keywords="nagios mysql slave replication monitoring",
     packages=find_packages(),
-    install_requires=["MySQL-python"],
+    install_requires=["mysqlclient"],
     entry_points={
         "console_scripts": ["check_mysql_slave=check_mysql_slave:main"],
     },