contrib: have check-code look at files in latin1 instead of ascii

This way all files open. So far none of our patterns look like they'll care.

Differential Revision: https://phab.mercurial-scm.org/D4249

contrib/check-code.py

@@ -30,7 +30,7 b' if sys.version_info[0] < 3:'
     opentext = open
 else:
     def opentext(f):
-        return open(f, encoding='ascii')
+        return open(f, encoding='latin1')
 try:
     xrange
 except NameError: