Add vat alias so stdnum.vatin works for the United States

While stdnum provides several modules implementing some sort of TIN for the
United States, EIN is the only only used for companies and institutions.

Author: unho

stdnum/us/__init__.py

@@ -19,3 +19,6 @@
 # 02110-1301 USA
 
 """Collection of United States numbers."""
+
+# provide aliases
+from stdnum.us import ein as vat  # noqa: F401