Revert "Insert 2to3 invocation into setup.py."

This reverts commit 4f913f6d7d4cb0bd5d6e4e09b322a4382c72badc,
in accordance with the decision [1] to not use 2to3.

[1] https://github.com/inducer/pudb/pull/1
Fix Python 2-ism in exception catching.

Proper Py2/3 fix pointed out by Aaron Meurer.