Update sphinx vesion to make it compatible with RTD

.readthedocs.yaml

@@ -0,0 +1,18 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+python:
+  version: 3.7
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs

setup.py

@@ -31,7 +31,16 @@
     ],
     install_requires=open('requirements.txt').read().strip().split('\n'),
     tests_require=test_pckgs,
-    extras_require={"test": test_pckgs},
+    extras_require={
+        "test": test_pckgs,
+        "docs": [
+            # pin sphinx to match what RTD uses:
+            # https://github.com/readthedocs/readthedocs.org/blob/ecac31de54bbb2c100f933e86eb22b0f4389ba84/requirements/pip.txt#L16
+            'sphinx<2',
+            'sphinx-rtd-theme<0.5',
+            'docutils<0.18'
+            ]
+        },
     package_data={
         'py_backend': [
             'xmile/smile.grammar'