Replace use of deprecated alias jinja2.Markup

- use `markupsafe.Markup` directly instead.

Note: the choice of version is to match https://github.com/pallets/jinja/blob/2.10.1/setup.py

requirements/project-min.txt

@@ -1,6 +1,7 @@
 babel==2.9.0
 click==3.3
 Jinja2==2.10.1
+MarkupSafe==0.23
 livereload==2.5.1
 Markdown==3.2.1
 PyYAML==5.1

requirements/project.txt

@@ -1,6 +1,7 @@
 babel>=2.9.0
 click>=7.0
 Jinja2>=2.10.3
+MarkupSafe>=0.23
 livereload>=2.6.1
 Markdown>=3.2.1
 PyYAML>=5.2