Attempt to fix RTD build

Author: mehaase

.readthedocs.yml

@@ -0,0 +1,11 @@
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+version: 2
+
+sphinx:
+  configuration: docs/conf.py
+
+python:
+  version: 3.7
+  pip_install: true
+  extra_requirements:
+    - docs

pyproject.toml

@@ -25,6 +25,11 @@ sphinx = "^3.0.1"
 sphinx-autodoc-typehints = "^1.10.3"
 sphinx-rtd-theme = "^0.4.3"
 
+# The docs extra is a temporary hack to build on RTD. See:
+# https://github.com/readthedocs/readthedocs.org/issues/4912#issuecomment-489818696
+[tool.poetry.extras]
+docs = ["sphinx", "sphinx-autodoc-typehints", "sphinx-rtd-theme"]
+
 [build-system]
 requires = ["poetry>=0.12"]
 build-backend = "poetry.masonry.api"