zeroconf: fix traceback under py3

hg serve under py3 caused
struct.error: char format requires a bytes object of length 1

<pulkit25> ah, I think that should be `pycompat.bytechr` instead of chr

hgext/zeroconf/Zeroconf.py

@@ -1191,7 +1191,11 b' class ServiceInfo(object):'
                 list.append(b'='.join((key, suffix)))
             for item in list:
                 result = b''.join(
-                    (result, struct.pack(b'!c', chr(len(item))), item)
+                    (
+                        result,
+                        struct.pack(b'!c', pycompat.bytechr(len(item))),
+                        item,
+                    )
                 )
             self.text = result
         else: