Bump to 2021.12.post0

This is due to a pipeline failure, and losing the release version on
PyPI afterwards.

Dockerfile

-ARG APPS="AlekSIS==2021.12"
+ARG APPS="AlekSIS==2021.12.post0"
 ARG BUILD_DEPS="libmariadb-dev gcc python3-dev"
 ARG SYSTEM_DEPS="default-mysql-client-core"
 

meta/aleksis-builddeps/pyproject.toml

 [tool.poetry]
 name = "AlekSIS-Builddeps"
-version = "2021.12"
+version = "2021.12.post0"
 packages = []
 
 description = "AlekSIS (School Information System) — Build/Dev dependencies for apps"

meta/aleksis/pyproject.toml

 [tool.poetry]
 name = "AlekSIS"
-version = "2021.12"
+version = "2021.12.post0"
 packages = [ { include = "aleksis_meta" } ]
 include = ["LICNECE.rst"]
 readme = "README.rst"