do not release if uncommitted changes

release.xsh

@@ -1,6 +1,8 @@
 #!/usr/local/bin/xonsh
 
 try:
+    no_uncommitted_changes = bool(!(git diff-index --quiet HEAD --))
+    if no_uncommitted_changes:
         pip install wheel twine
         python setup.py sdist bdist_wheel
         twine upload dist/*