check-code: use raw string

This avoids a SyntaxWarning in Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D5829

contrib/check-code.py

@@ -414,7 +414,7 b' txtfilters = []'
 
 txtpats = [
   [
-    ('\s$', 'trailing whitespace'),
+    (r'\s$', 'trailing whitespace'),
     ('.. note::[ \n][^\n]', 'add two newlines after note::')
   ],
   []