Fixed doc build issue with jinja

docs/requirements.txt

@@ -1,8 +1,9 @@
 # Latest versions at time of writing.
-mkdocs==1.2.3            # Main documentation generator.
+mkdocs==1.3.0            # Main documentation generator.
 mkdocs-material==7.3.6   # Theme
 pymdown-extensions==9.1  # Markdown extensions e.g. to handle LaTeX.
 mkdocstrings==0.17.0     # Autogenerate documentation from docstrings.
 mknotebooks==0.7.1       # Turn Jupyter Lab notebooks into webpages.
 pytkdocs_tweaks==0.0.4   # Tweaks mkdocstrings to improve various aspects
 mkdocs_include_exclude_files==0.0.1  # Allow for customising which files get included
+jinja2==3.0.3            # Older version. After 3.1.0 seems to be incompatible with current versions of mkdocstrings.