Merge #2437

2437: Fix mysql r=mergify[bot] a=nextgens

## What type of PR?

bug-fix

## What does this PR do?

In #2422 we have switched from mysqlclient to mysql-connector ... and apparently SQLalchemy needs to be told explicitly.

This hasn't been tested.

### Related issue(s)
- close #2435

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [ ] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>

core/admin/mailu/configuration.py

@@ -98,7 +98,7 @@ class ConfigManager:
     DB_TEMPLATES = {
         'sqlite': 'sqlite:////{SQLITE_DATABASE_FILE}',
         'postgresql': 'postgresql://{DB_USER}:{DB_PW}@{DB_HOST}/{DB_NAME}',
-        'mysql': 'mysql://{DB_USER}:{DB_PW}@{DB_HOST}/{DB_NAME}'
+        'mysql': 'mysql+mysqlconnector://{DB_USER}:{DB_PW}@{DB_HOST}/{DB_NAME}'
     }
 
     def __init__(self):

towncrier/newsfragments/2432.bugfix

@@ -0,0 +1 @@
+Switch from mysqlclient to mysql-connector explicitely