Now request specific Python package versions in docs/requirements.txt

Using upper bounds in the pip "requirements.txt" file for ReadTheDocs
may not necessarily force installation of the desired package version
(e.g. if there is already a version present that satisfies the
constraints).

We have now modified docs/requirements.txt to request specific package
versions, which should avoid this ambiguity.

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

docs/requirements.txt

@@ -1,15 +1,10 @@
 # Requirements for building the KPP documentation
 #
-# NOTE: There are issues with sphinx 4.0.0+ and sphinx-rtd-theme 1.0.0
-# so we'll use older versions (Bob Yantosca, 03 May 2022)
+# These Python package versions have been proven to work:
 #
-# ALSO NOTE: A recent update to jinja2 has broken backwards c
-# compatibility with the older ReadTheDocs package, so we need to
-# use a version of jinja2 prior to 3.1.0. (Bob Yantosca, 03 May 2022)
-#
-sphinx<4.0.0
-sphinx_rtd_theme<1.0.0
+sphinx==3.5.4
+sphinx_rtd_theme==0.5.2
 sphinxcontrib-bibtex==2.2.0
 recommonmark
 docutils==0.16
-jinja2<3.1.0
+jinja2==3.0.3