Use utf8mb4 as pymysql client charset in default env

[janbrasna]

When investigating #341 using the boilerplate .env-dist connection strings locally, it shows that the defaults used for connecting to mariadb would trigger DataError ("Incorrect string value") exception with certain content posted:

It turns out I was able to post either Latin-ext or non-Latin Unicode scripts successfully, or use e.g. a heart ❤️ emoji — however the app would crash if I tried anything more astral as facepalming 🤦 monkeys 🙈 et al. …

(passed:)

(crashed:)

The default charset=utf8 used is actually utf8mb3, not full utf8mb4, so that effectively rules out emoji etc.:/ — TL;DR: https://forum.djangoproject.com/t/ticket-18392-and-mysql-utf8mb4/34916

This won't magically fix production tables and collation for mb4 storage, but is currently needed for the Django versions used to at least stop crashing on such content locally.

Passing this arg to dj_database_url here expands it into OPTIONS correctly, as can be seen below running this tweak locally and being able to insert emoji: … ;)