[ci][docs] Fix RTD builds (#3015)

* hotfix for pip error

* Update conf.py

* Update conf.py

* Update conf.py

* fix rtd builds

.readthedocs.yml

@@ -4,7 +4,7 @@ formats:
 python:
   version: 3
   install:
-    - requirements: docs/requirements.txt
+    - requirements: docs/requirements_rtd.txt
 sphinx:
   builder: html
   configuration: docs/conf.py

docs/requirements_rtd.txt

@@ -0,0 +1,4 @@
+sphinx >= 3.0.2
+sphinx_rtd_theme >= 0.3
+mock; python_version < '3'
+breathe