Remove Python 3.3 from testing.

- Support for Python 3.3 isn't really needed, as nothing uses it as the
  default system Python, and nearly everyone will have a newer Python 3
  version installed.

.travis.yml

@@ -35,10 +35,6 @@ jobs:
       os: linux
       language: python
       env: TEST_SUITE=unit
-    - python: '3.3'
-      os: linux
-      language: python
-      env: TEST_SUITE=unit
     - python: '3.4'
       os: linux
       language: python

lib/spack/docs/contribution_guide.rst

@@ -56,7 +56,7 @@ 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, 2.7, and 3.3-3.6 on both macOS and Linux. We currently
+against Python 2.6, 2.7, and 3.4-3.7 on both macOS and Linux. We currently
 perform 3 types of 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 (2.6 or 2.7) or 3 (3.3 - 3.6)
+1. Python 2 (2.6 or 2.7) or 3 (3.4 - 3.7)
 2. A C/C++ compiler
 3. The ``git`` and ``curl`` commands.
 4. If using the ``gpg`` subcommand, ``gnupg2`` is required.