Update py-setuptools to 20.9.0.

v20.9.0
-------

* #548: Update certify version to 2016.2.28
* #545: Safely handle deletion of non-zip eggs in rotate
  command.

v20.8.1
-------

* Issue #544: Fix issue with extra environment marker
  processing in WorkingSet due to refactor in v20.7.0.

v20.8.0
-------

* Issue #543: Re-release so that latest release doesn't
  cause déjà vu with distribute and setuptools 0.7 in
  older environments.

v20.7.0
-------

* Refactored extra enviroment marker processing
  in WorkingSet.
* Issue #533: Fixed intermittent test failures.
* Issue #536: In msvc9_support, trap additional exceptions
  that might occur when importing
  ``distutils.msvc9compiler`` in mingw environments.
* Issue #537: Provide better context when package
  metadata fails to decode in UTF-8.

v20.6.8
-------

* Issue #523: Restored support for environment markers,
  now honoring 'extra' environment markers.

(wiz)