Move Open Source to 20.12.2 to release

Summary:
- Update setup.py to release a new version
- This will get all of 2020's fixes to xar

Reviewed By: terrelln

Differential Revision: D25279244

fbshipit-source-id: 12d3d550f3d254d74c9f5bd5af50991498c85ba1

setup.py

@@ -17,7 +17,7 @@ def get_long_description():
 
 setup(
     name="xar",
-    version="19.4.22",
+    version="20.12.2",
     description="The XAR packaging toolchain.",
     long_description=get_long_description(),
     long_description_content_type="text/markdown",