Updates to the documentation tech stack have resulted in errors and t…

…he docs could not be built.

This change introduces a workaround, however there are still warnings being raised. It is likely
that they will persist until the underlying libraries adapt to the new versions. This commit
should restore the ability for readthedocs to build the documentation (hopefully).

docs/conf.py

@@ -54,7 +54,7 @@
 # -- nbsphinx ---------------------------------------------------------------
 # This is processed by Jinja2 and inserted after each notebook
 nbsphinx_prolog = r"""
-{% set docname = '' + env.doc2path(env.docname, base=False) %}
+{% set docname = '' + env.doc2path(env.docname, base=False)|string() %}
 
 .. raw:: html
 
@@ -73,7 +73,7 @@
 
 # This is processed by Jinja2 and inserted after each notebook
 nbsphinx_epilog = r"""
-{% set docname = 'doc/' + env.doc2path(env.docname, base=None) %}
+{% set docname = 'doc/' + env.doc2path(env.docname, base=None)|string() %}
 .. raw:: latex
 
     \nbsphinxstopnotebook{\scriptsize\noindent\strut