Declare support for Python 3.6

Per the [changelog](https://github.com/pypa/virtualenv/blob/6d8a4ed81f40223a3f9ea1ba5d11be500d9f667a/docs/changes.rst)
notes for `15.1.0`, Python 3.6 is supported. This change adds 3.6 to the
trove classifiers and drops the now-unsupported 3.3.
2 files changed