Now tests all supported Python environments as well. Allows testing the
full matrix of Python version with a single command.
Include the command make in the whitelist_externals configuration to
avoid the runtime warning:
WARNING:test command found but not installed in testenv
cmd: /usr/bin/make
env: .../psycopg2/.tox/flake8
Maybe you forgot to specify a dependency? See also the whitelist_externals envconfig setting.