Update py-blinker to 1.3.

Version 1.3
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Released July 3, 2013

- The global signal stash behind blinker.signal() is now backed by a
  regular name-to-Signal dictionary. Previously, weak references were
  held in the mapping and ephermal usage in code like
  ``signal('foo').connect(...)`` could have surprising program behavior
  depending on import order of modules.
- blinker.Namespace is now built on a regular dict. Use
  blinker.WeakNamespace for the older, weak-referencing behavior.
- Signal.connect('text-sender') uses an alterate hashing strategy to
  avoid sharp edges in text identity.

(kleink)