Update database drivers #1355

We pin the version on 3 database-specific (MySQL) packages. They are all getting long in the tooth. Updating them could give us better performance, but might also cause problems or require non-trivial changes:

• delphi-epidata/requirements.api.txt

  Line 7 in bc7e375

  mysqlclient==2.1.1

  • 1.5y old
  • https://pypi.org/project/mysqlclient/#history

• delphi-epidata/requirements.dev.txt

  Line 16 in bc7e375

  pymysql==1.0.2

  • almost 3y old!
  • https://pypi.org/project/pymysql/#history

• delphi-epidata/requirements.dev.txt

  Line 13 in bc7e375

  mysql-connector==2.2.9

  • ~4.5y old!!!
  • was: https://pypi.org/project/mysql-connector/#history
  • replaced by: https://pypi.org/project/mysql-connector-python/#history