py3: system-stringify file mode in commandserver.py

# skip-blame just r'' prefixes

mercurial/commandserver.py

@@ -344,8 +344,8 b' def _initworkerprocess():'
     random.seed()
 
 def _serverequest(ui, repo, conn, createcmdserver):
-    fin = conn.makefile('rb')
-    fout = conn.makefile('wb')
+    fin = conn.makefile(r'rb')
+    fout = conn.makefile(r'wb')
     sv = None
     try:
         sv = createcmdserver(repo, conn, fin, fout)