setup.py: require pytest-runner only when necessary (#119)

This optimizes setup.py for cases when pytest-runner is not needed,
using the approach that is suggested upstream:

https://pypi.python.org/pypi/pytest-runner#conditional-requirement

setup.py

@@ -1,9 +1,14 @@
 #!/usr/bin/env python
 import re
+import sys
 
 from setuptools import find_packages, setup
 
 
+needs_pytest = set(['pytest', 'test', 'ptr']).intersection(sys.argv)
+pytest_runner = ['pytest-runner'] if needs_pytest else []
+
+
 # Get version without importing, which avoids dependency issues
 def get_version():
     with open('chardet/version.py') as version_file:
@@ -46,7 +51,7 @@ def readme():
                     "Modules"),
                    "Topic :: Text Processing :: Linguistic"],
       packages=find_packages(),
-      setup_requires=['pytest-runner'],
+      setup_requires=pytest_runner,
       tests_require=['pytest', 'hypothesis'],
       entry_points={'console_scripts':
                     ['chardetect = chardet.cli.chardetect:main']})