Avoid importing pip

pip doesn't have a stable Python API and shouldn't be `import`ed. `pkg_resources` is provided by setuptools.

setup.py

@@ -1,10 +1,10 @@
 #!/usr/bin/env python
 from setuptools import setup, find_packages
-import pip
+import pkg_resources
 import sys
 import os
 
-if int(pip.__version__.split('.')[0]) < 6:
+if int(pkg_resources.get_distribution("pip").version.split('.')[0]) < 6:
     print('pip older than 6.0 not supported, please upgrade pip with:\n\n'
           '    pip install -U pip')
     sys.exit(-1)