Ensure Python3 before reading README with encoding arg

If you accidently try to install using Python 2 you get the error

    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/home/nokome/nokome/repo2docker/setup.py", line 5, in <module>
        with open('README.md', encoding="utf8") as f:
    TypeError: 'encoding' is an invalid keyword argument for this function

Ideally the user would be aked to use Python 3 as `python_requires` tells us,
but which isn't reached yet.
This implements that behaviour.

setup.py

@@ -1,6 +1,10 @@
 from setuptools import setup, find_packages
 import versioneer
 
+import sys
+if sys.version_info[0] < 3:
+    sys.stderr.write('jupyter-repo2docker requires Python 3 but the running Python is %s.%s.%s' % sys.version_info[:3])
+    sys.exit(1)
 
 with open('README.md', encoding="utf8") as f:
     readme = f.read()