fix warnings for Python 3.7 to 3.9

Author: boisgera

setup.py

@@ -37,7 +37,7 @@
 # ------------------------------------------------------------------------------
 metadata = dict(
   name = "bitstream",
-  version = "2.6.0.dev0",
+  version = "2.6.0",
   description = "Binary Data for Humans",
   url = "https://github.com/boisgera/bitstream",
   author = u"Sébastien Boisgérault",