Explicitly specify RTD formats (#196)

There is a new RTD format readthedocssinglehtmllocalmedia which causes furo to choke.
Limiting formats to htmlzip, pdf, and epub for now.

Author: speedyleion

.readthedocs.yml

@@ -13,7 +13,10 @@ sphinx:
   fail_on_warning: false
 
 # Optionally build your docs in additional formats such as PDF and ePub
-formats: all
+formats: 
+  - htmlzip
+  - pdf
+  - epub
 
 # Optionally set the version of Python and requirements required to build your
 # docs