reset sphinx version

Author: tyeth

.github/workflows/docs.yml

@@ -13,7 +13,7 @@ on:
       build-python-version:
         description: Python version used for docs build
         type: string
-        default: 3.6
+        default: '3.6'
 
 jobs:
   build_pages:

docs/requirements.txt

@@ -1,7 +1,7 @@
-sphinx>=3.0
-sphinx_rtd_theme
+sphinx~=1.8.3
+sphinx_rtd_theme~=0.4.2
 
 # Unfortunately the pip package "sphinxcontrib-versioning" is
 # broken and not maintained anymore, thus cloning it directly from
 # a forked Git repository which contains important bugfixes.
-# git+https://github.com/leokoppel/sphinxcontrib-versioning.git@93fe26211de4322c0ddf347f51e095b580cdcd55
+git+https://github.com/leokoppel/sphinxcontrib-versioning.git@93fe26211de4322c0ddf347f51e095b580cdcd55