Fix ResourceWarning due to unclosed file resource.

Author: tirkarthi

importlib_resources/_common.py

@@ -203,5 +203,6 @@ def _write_contents(target, source):
         for item in source.iterdir():
             _write_contents(child, item)
     else:
-        child.open('wb').write(source.read_bytes())
+        with child.open('wb') as fp:
+            fp.write(source.read_bytes())
     return child