Try to fix rtfd

Author: jnothman

doc/conf.py

@@ -26,6 +26,8 @@
 # project root
 sys.path.insert(0, os.path.abspath('..'))
 
+os.environ['MPLBACKEND'] = 'Agg'  # avoid tkinter import errors on rtfd.io
+
 # -- General configuration ---------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.