--- a
+++ b/docs/requirements.txt
@@ -0,0 +1,6 @@
+# Requirements file for ReadTheDocs, check .readthedocs.yml.
+# To build the module reference correctly, make sure every external package
+# under `install_requires` in `setup.cfg` is also listed here!
+# sphinx_rtd_theme
+recommonmark
+sphinx>=3.2.1