Config: declare type of the db_index_url attribute.

mypy (and likely other static analysis tools) will complain if an
object attribute is used without being defined either in the class or
the class's __init__ method.  Declare the attribute here so that mypy
knows it exists.

Author: Benjamin Moody

wfdb/io/download.py

@@ -23,7 +23,7 @@ class Config(object):
 
     """
 
-    pass
+    db_index_url: str
 
 
 # The configuration database index url. Uses PhysioNet index by default.