switch back to non-conda based rtd config

 * ReadTheDocs has updated their base python image, meaning the
   anaconda-based config is no longer needed. The conda config was
   out-of-date and failing to build anyway since the setuptools
   dependency version bump.

Author: felliott

environment.yml

@@ -1,2 +1,3 @@
-conda:
-    file: environment.yml
+requirements_file: doc-requirements.txt
+python:
+   setup_py_install: true