Remove find_packages(); use setuptools.PEP420PackageFinder.find()

`PEP420PackageFinder.find()` does basically the same the tangled version
did, except that the former doesn't automatically exclude anything, but
that's no biggie.
In release script, run setup() programmatically

Previously, the release script would run `python setup.py sdist ...` in
a subprocess. Now, it use `exec()` to run load/run setup.py. This way,
the setup script is run in the same environment that the release script
is run in.