tests: in filterpyflakes, tolerate non-ascii file contents

tests/filterpyflakes.py

@@ -24,10 +24,9 b' for line in sys.stdin:'
             break  # pattern matches
     if keep:
         fn = line.split(':', 1)[0]
-        f = open(fn)
-        data = f.read()
-        f.close()
-        if 'no-' 'check-code' in data:
+        with open(fn, 'rb') as f:
+            data = f.read()
+        if b'no-' b'check-code' in data:
             continue
         lines.append(line)