Update mkdocs to 1.3.x

Author: nezhar

requirements/requirements-documentation.txt

@@ -1,5 +1,5 @@
 # MkDocs to build our documentation.
-mkdocs>=1.1.2,<1.2
+mkdocs>=1.3,<1.4
 jinja2>=2.10,<3.1.0 # contextfilter has been renamed
 
 # pylinkvalidator to check for broken links in documentation.