Exclude the to environment

This suppresses warnings about ‘system’ modules not having docs.

conf.py

@@ -65,7 +65,7 @@
 
 # List of directories, relative to source directory, that shouldn't be searched
 # for source files.
-exclude_patterns = ['_build/*']
+exclude_patterns = ['_build/*', '.tox/*']
 
 # The reST default role (used for this markup: `text`) to use for all documents.
 #default_role = None