Import setuptools for install_requires.

install_requires is a setuptools option, and using it with distutils will
only trigger a warning or a fatal error (in Python 3.3).
Make pymaging test optional.

Skip the pymaging test if pymaging-png is not installed.
Fix invalid alphanumeric qr codes due to incorrect regex

Fixes #44
Merge pull request #41 from mgorny/tests-without-pymaging

Support using either pymaging or PIL for tests
More work towards setuptools simplification

Fixes #42
Prep for 4.0.2 release