Remove Python 3.3 support. (#1031)

* Remove Python 3.3 support.
Redis >= 3.X does only support Python >=3.4.
Python 3.7 is no longer dev.

* Use dev for Python 3.7 build again.

.travis.yml

@@ -4,7 +4,6 @@ services:
   - redis
 python:
   - "2.7"
-  - "3.3"
   - "3.4"
   - "3.5"
   - "3.6"

setup.py

@@ -68,7 +68,6 @@ setup(
         'Programming Language :: Python :: 2',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',