Provide a version / version_info module constant

[giampaolo]

From g.rodola on August 01, 2012 00:07:49

...In order to allow version checking to figure whether we're using deprecated 1.2.1 version: https://code.google.com/p/pyftpdlib/issues/detail?id=211#c1

Original issue: http://code.google.com/p/pysendfile/issues/detail?id=17