Pin specific version of Jinja2

Before this change, the docs failed to build with the following
error:

ImportError: cannot import name 'environmentfilter' from 'jinja2'

A fix to this problem is to explicitly pin Jinja2 version.

Author: avelanarius

docs/pyproject.toml

@@ -14,6 +14,7 @@ sphinx-sitemap = "2.1.0"
 sphinx-autobuild = "0.7.1"
 Sphinx = "2.4.4"
 sphinx-multiversion-scylla = "~0.2.6"
+Jinja2 = "<3.1"
 
 [tool.poetry.dev-dependencies]
 pytest = "5.2"