do not release if uncommitted changes

release.xsh

@@ -1,9 +1,11 @@
 #!/usr/local/bin/xonsh
 
 try:
-    pip install wheel twine
-    python setup.py sdist bdist_wheel
-    twine upload dist/*
+    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/*
 
 finally:
     rm -rf dist/ build/