[sphinx] Update conf.py for Python 3 compatibility (SR-1763). (#3086)

Patch by Gwynne Raskind.

https://bugs.swift.org/browse/SR-1763

Author: jrose-apple

docs/conf.py

@@ -42,7 +42,7 @@
 
 # General information about the project.
 project = u'Swift'
-copyright = unicode(date.today().year) + u', Apple Inc'
+copyright = str(date.today().year) + u', Apple Inc'
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the