validation: fix nosetest in gyb for Python 3

The file-like object's `write` method in Python 3 returns the number of
bytes that are written.  This was not previously checked and is not
particularly interesting.  Simply blackhole the bytes written, repairing
the test on Python 3.

Author: compnerd

utils/gyb.py

@@ -763,8 +763,8 @@ def expand(filename, line_directive=_default_line_directive, **local_bindings):
     >>> # manually handle closing and deleting this file to allow us to open
     >>> # the file by its name across all platforms.
     >>> f = NamedTemporaryFile(delete=False)
-    >>> f.write(
-    ... r'''---
+    >>> _ = f.write(
+    ... br'''---
     ... % for i in range(int(x)):
     ... a pox on ${i} for epoxy
     ... % end