Explicitly define python version for building docs and deps

Author: martyanov

.readthedocs.yml

@@ -1,5 +1,6 @@
 version: 2
 python:
+  version: 3.8
   install:
     - method: pip
       path: .

setup.py

@@ -48,14 +48,16 @@ def _get_long_description():
     extras_require={
         'dev': [
             'PyYAML==5.3.1',
-            'Sphinx==3.1.0',
             'coverage==5.1',
             'flake8-docstrings==1.3.0',
             'flake8-import-order==0.18.1',
             'flake8==3.8.3',
             'grpcio-tools==1.29.0',
             'twine==3.1.1',
         ],
+        'doc': [
+            'Sphinx==3.1.0',
+        ],
         'test': [
             'pifpaf==2.5.0',
             'pytest-asyncio==0.10.0',