issue #768: use feature detection for Compress.copy()

Work around a seemingly impossible condition by feature detecting
zlib.compressobj().copy rather than assuming it is available in all
Python releases. The related ticket looks possibly like some weird
vendor-patched version of Python in use, which has happened in the past
particularly on Red Hat.

mitogen/parent.py

@@ -692,9 +692,11 @@ class PartialZlib(object):
     A full compression costs ~6ms on a modern machine, this method costs ~35
     usec.
     """
+    HAS_COPY = hasattr(zlib.compressobj(), 'copy')
+
     def __init__(self, s):
         self.s = s
-        if sys.version_info > (2, 5):
+        if self.HAS_COPY:
             self._compressor = zlib.compressobj(9)
             self._out = self._compressor.compress(s)
             self._out += self._compressor.flush(zlib.Z_SYNC_FLUSH)