Improve compatibility with Python 2.6

Keyword arguments for `decode` are only supported since 2.7

src/potr/proto.py

@@ -123,7 +123,7 @@ def __init__(self, versions=set()):
     def parse(cls, data):
         if not isinstance(data, bytes):
             raise TypeError('can only parse bytes')
-        udata = data.decode('ascii', errors='replace')
+        udata = data.decode('ascii', 'replace')
 
         versions = set()
         if len(udata) > 0 and udata[0] == '?':