Try to fix docs dependencies

Docs were not building on read the docs, see also: https://github.com/sphinx-doc/sphinx/issues/9788

.readthedocs.yml

@@ -1,2 +1,8 @@
+version: 2
+
 python:
-   pip_install: true
+   install:
+     - method: pip
+       path: .
+       extra_requirements:
+         - docs

setup.py

@@ -46,6 +46,10 @@ setup(name='jedi',
               'flake8==3.8.3',
               'mypy==0.782',
           ],
+          'docs': [
+              'sphinx<2',
+              'docutils<0.18',
+          ],
       },
       package_data={'jedi': ['*.pyi', 'third_party/typeshed/LICENSE',
                              'third_party/typeshed/README']},