https://bugs.mysql.com/bug.php?id=91828
https://github.com/aio-libs/aiomysql/issues/85

Protocol change or connector version? Will try on a buster host.

root@db1114:~$ python3
Python 3.7.3 (default, Dec 20 2019, 18:57:59) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from WMFMariaDB import WMFMariaDB
>>> db=WMFMariaDB('db1114', debug=True)
Connected to db1114/(none)
>>> db.execute('SELECT 1')
Executing 'SELECT 1'
{'query': 'SELECT 1', 'host': 'db1114.eqiad.wmnet', 'port': 3306, 'database': None, 'success': True, 'numrows': 1, 'rows': ((1,),), 'fields': ('1',)}

Buster libs seem to work. But mariadb client doesn't work either, you have to connect with the percona/mysql client. I will double check it is not specific to this user I created, but it seems it requires cumin to be upgraded for the new client library to work:

python3-pymysql
Buster:
Version: 0.9.3-1

Stretch:
Version: 0.7.10-1

The Cumin update is something that will happen next Q, but if you need it earlier, we can backport it as well?

Not worth the work. This is not blocking anything important- that is why I just closed it. This was a FYI, to both cumin and/or deb monitor owners. This was the first time I realized this incompatibility on this connector, so I thought it was important to make it aware beyond DBAs.