Bump pymysql version

[nick-mccoy]

Description of change

Updating pymysql version from 0.7.11 to 0.9.3

The two (documented) backwards incompatible changes from the PyMySQL Changelog are:

1. binary_prefix option is added and off by default
2. multi_statements flag is no longer set by default

Neither of these changes affect the tap.

The two changes made to the tap are:

1. Monkey patch PyMySQL's convert_date and convert_timestamp to return None instead of the string 0000-00-00 00:00:00 to keep behavior consistent with 0.7.11 since it was changed in this PyMySQL commit
2. Remove a patch we had applied to mimic the behavior of this PyMySQL PR for support of unknown MySQL 8 charsets

QA steps

• automated tests passing
• Added a test to ensure that zero datetimes come back as None
• manual qa steps passing (list below)

Risks

Any undocumented, backwards incompatible changes added to PyMySQL between 0.7.11 and 0.9.3

Rollback steps

• revert this branch