Error in migration_db.py: 'MySQLdb' is not defined

[oplantalech]

@zainasir I think the error comes from #83

[zainasir]

@oplantalech That is weird because #83 was supposed to fix this issue. Me and @dippindots spent a lot of time on this and realized that manually importing MySQLdb fixed it. Our CI setups have also been working correctly after this PR so I am a little confused why you are still seeing that error.

[oplantalech]

The issue is being taking care in cBioPortal/cbioportal#11632