Spack works with Python 3

- Update version guard in spack script to allow python 3
- Update min required version in the docs

lib/spack/docs/contribution_guide.rst

@@ -40,7 +40,7 @@ for the results of these tests after submitting a PR, we recommend that you run
 locally to speed up the review process.
 
 If you take a look in ``$SPACK_ROOT/.travis.yml``, you'll notice that we test
-against Python 2.6 and 2.7. We currently perform 3 types of tests:
+against Python 2.6, 2.7, and 3.3-3.6. We currently perform 3 types of tests:
 
 ^^^^^^^^^^
 Unit Tests

lib/spack/docs/getting_started.rst

@@ -11,7 +11,7 @@ Prerequisites
 Spack has the following minimum requirements, which must be installed
 before Spack is run:
 
-1. Python 2.6 or 2.7
+1. Python 2 (2.6 or 2.7) or 3 (3.3 - 3.6)
 2. A C/C++ compiler
 3. The ``git`` and ``curl`` commands.