Updated Sphinx source_suffix setting to use a mapping.

Since Sphinx 1.8 this setting should be a mapping of file extensions to
file types. Before this change, Sphinx 8+ would show the following  when
building docs:

Converting `source_suffix = '.txt'` to `source_suffix = {'.txt': 'restructuredtext'}`

docs/conf.py

@@ -94,7 +94,7 @@ spelling_warning = True
 # templates_path = []
 
 # The suffix of source filenames.
-source_suffix = ".txt"
+source_suffix = {".txt": "restructuredtext"}
 
 # The encoding of source files.
 # source_encoding = 'utf-8-sig'