Fix upload_index_network_cached for python3 compatibility.

src/zc/buildout/networkcache.py

@@ -240,7 +240,7 @@ def upload_index_network_cached(dir_url, cache_url, external_url, base, requirem
     directory_key = get_index_directory_key(external_url, requirement)
     kw = dict(file="file",
               base=base,
-              urlmd5=hashlib.md5(external_url).hexdigest(),
+              urlmd5=hashlib.md5(external_url.encode()).hexdigest(),
               requirement=requirement)
 
     import tempfile