Remove python 3.3 Travis build

Stability of the Python 3.3 builds have declined over the last year
as more and more of our testing dependencies (e.g., tox) deprecate
or remove support for the version.

.travis.yml

@@ -1,21 +1,13 @@
 language: python
 python:
   - '2.7'
-  - '3.3'
   - '3.4'
   - '3.5'
   - '3.6'
   - '3.6-dev'
   - '3.7-dev'
 before_script:
-  - |
-    if [[ $TRAVIS_PYTHON_VERSION == 3.3 ]]; then
-      # virtualenv 16.0.0 bundles wheel 0.31.1, which drops python 3.3.* support
-      # tox 3.0.0 drops python 3.3.* support
-      pip install virtualenv==15.2.0 tox==2.9.1
-    else
-      pip install tox
-    fi
+  - pip install tox
   - if [[ $TRAVIS_PYTHON_VERSION == 3.6 ]]; then pip install flake8 flake8-bugbear; fi
 script:
   - tox -e py