Revert "Explicitly specify RTD formats (#196)"

This reverts commit e51291e.

This didn't solve the RTD build issue, reverting for now.

Author: speedyleion

.readthedocs.yml

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