codecs.open => io.open
codecs.open => io.open

because `codecs.open` is or will at some point be deprecated.

http://bugs.python.org/issue8796

Cc: da39a3ee5e6b4b0d3255bfef95601890afd80709@berkerpeksag

mgedmin/readme,pypa/readme,pypa/readme_renderer,sigmavirus24/readme