fix hdmrd5 check of local cached files

Current OBS is delivering hdrmd5 in buildinfo. It turns out
that osc has already code for validating cached files, but it
invalidates all local files atm with python 3.x

osc/util/packagequery.py

@@ -87,7 +87,7 @@ class PackageQuery:
         f = open(filename, 'rb')
         magic = f.read(7)
         f.seek(0)
-        if magic[:4] == '\xed\xab\xee\xdb':
+        if magic[:4] == b'\xed\xab\xee\xdb':
             from . import rpmquery
             f.close()
             return rpmquery.RpmQuery.queryhdrmd5(filename)