py3: silence f.write() in test-annotate.t

py3: silence f.write() in test-annotate.t

--- a/tests/test-annotate.t	Fri Mar 09 14:52:36 2018 -0800
+++ b/tests/test-annotate.t	Sat Mar 10 16:55:54 2018 +0900
@@ -914,10 +914,10 @@
   > EOF
 
   >>> with open('a', 'wb') as f:
-  ...     f.write(b'0a\r0b\r\n0c\r0d\r\n0e\n0f\n0g')
+  ...     f.write(b'0a\r0b\r\n0c\r0d\r\n0e\n0f\n0g') and None
   $ hg ci -qAm0
   >>> with open('a', 'wb') as f:
-  ...     f.write(b'0a\r0b\r\n1c\r1d\r\n0e\n1f\n0g')
+  ...     f.write(b'0a\r0b\r\n1c\r1d\r\n0e\n1f\n0g') and None
   $ hg ci -m1
 
   $ hg annotate -r0 a | $PYTHON "$TESTTMP/substcr.py"