Update to django 3.x

[Xiphoseer]

Packages that depend on Django

The following list is the current contents of our requirements.txt. A checked box means that the selected version is compatible with the version we are targeting in this PR. As a first step, that target is 3.0.11.

• django-cms==3.8.0 (3.0, 3.1)
• djangocms-bootstrap4==2.0.0 (3.0, 3.1)
• djangocms-history==2.0.0 (3.0, 3.1)
• djangocms-link==3.0.0 (3.0, 3.1)
• djangocms-redirect==0.6.0 (3.0, 3.1)
• djangocms-style==3.0.0 (3.0, 3.1)
• djangocms-text-ckeditor==4.0.0 (3.0, 3.1)
• djangocms-column==1.11.0 (<=3.2 in setup.py)
• django-auth-ldap==2.2.0 (3.0)
• django-redis==4.12.1 (3.0)
• django-select2==7.4.2 (3.0)
• djangocms-cascade==1.3.4 (3.0)
• django-git-version==0.1.0 (tiny, self-maintained)
• git+https://github.com/andersinno/django-rss-plugin (last commit on 13 Aug 2019, no explicit support for 3.x)

Packages that we'll drop

• django-sslserver==0.22 (local dev tool, missing but the last commit is about v3.x compat; do we even need this without local TUID testing?)
• git+https://github.com/isotoma/djangocms-inherit (dropped official support, no explicit support for 3.x)
• git+https://github.com/pawelmarkowski/cmsplugin-filer (dropped official support, no explicit support for 3.x)

Packages that we/django depend on

• mysqlclient==2.0.1
• pytz==2020.4
• bleach==3.2.1