trick rtd build environment

.readthedocs.yml

@@ -0,0 +1,9 @@
+# ReadTheDocs environment customization to allow us to use conda to install
+# libraries which have C dependencies for the doc build. See:
+# http://docs.readthedocs.io/en/latest/yaml-config.html
+
+conda:
+    file: .rtd-environment.yml
+
+python:
+   setup_py_install: true

.rtd-environment.yml

@@ -0,0 +1,8 @@
+# Conda environment file used by our custom ReadTheDocs environment.
+name: buzzard
+dependencies:
+    - python>=3.6
+    - numpy>=1.13
+    - scipy>=0.19.1
+    - gdal>=2.1.3
+    - shapely>= 1.5.16