Update requirements for ReadTheDocs; use jinja2 prior to 3.1.0

A Python package update has broken backwards compatibility with the
ReadTheDocs theme versions prior to 1.0.0.  In order to fix this,
we now specify that we should use a version of jinja2 prior to 3.1.0.

Also, for safety's sake, request specific versions of other python
packages that have been proven to build the documentation properly.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

docs/requirements.txt

@@ -1,6 +1,15 @@
 # Requirements for building the GCHP documentation
-sphinx
-sphinx_rtd_theme
-sphinxcontrib-bibtex 
+#
+# NOTE: There are issues with sphinx 4.0.0+ and sphinx-rtd-theme 1.0.0
+# so we'll use older versions (Bob Yantosca, 01 Feb 2022)
+#
+# Also note: a recent update to jinja2 has broken backwards
+# compatibility with the older ReadTheDocs package, so we need to
+# use a version of jinja2 prior to 3.1.0. (Bob Yantosca, 02 May 2022)
+#
+sphinx<4.0.0
+sphinx_rtd_theme<1.0.0
+sphinxcontrib-bibtex==2.2.0
 recommonmark
-docutils==0.16   # temporary fix for unordered lists not rendering properly
+docutils==0.16
+jinja2<3.1.0