Only make .tar.gz sdists when releasing

PEP 527, which was just accepted, says that there can only be one sdist
per release. Tarballs are already the more common sdist format on PyPI,
so let's go with that.

tools/toollib.py

@@ -18,7 +18,7 b" archive = '%s:%s' % (archive_user, archive_dir)"
 
 # Build commands
 # Source dists
-sdists = './setup.py sdist --formats=gztar,zip'
+sdists = './setup.py sdist --formats=gztar'
 # Binary dists
 def buildwheels():
     sh('python3 setupegg.py bdist_wheel' % py)