zeroconf: fix an issue concatenating bytes and str

`bytes(length)` doesn't do what we want, so use `str`.

There appear to be a ton more issues in this extension, including:

    - globals()[b'_GLOBAL_DONE'] using bytes as the key
    - `__author__` and similar using bytes
    - `BadDomainName` is feeding bytes to the Exception constructor
    - DNSRecord.toString() has the wrong signature (should be str, not bytes)

Differential Revision: https://phab.mercurial-scm.org/D11303

hgext/zeroconf/Zeroconf.py

@@ -770,7 +770,7 b' class DNSOutgoing(object):'
 
     def writeString(self, value, length):
         """Writes a string to the packet"""
-        format = b'!' + str(length) + b's'
+        format = '!' + str(length) + 's'
         self.data.append(struct.pack(format, value))
         self.size += length