External wheel builder package env

[gaborbernat]

To replace https://github.com/tox-dev/tox-external-wheels and exist alongside https://tox.readthedocs.io/en/rewrite/cli_interface.html#tox-run---installpkg

[testenv]
package = external

[testenv:.pkg]
package_glob = *.whl
commands = 
	pyproject-build -w . -o {envtmpdir}