Close README.rst after reading it
Close README.rst after reading it

This minor change fixes a warning when using pypy which complains with a
ResourceWarning:

    ResourceWarning: unclosed file <_io.TextIOWrapper name='<snip>/netifaces-0.10.6/README.rst' mode='r' encoding='UTF-8'>

al45tair/netifaces,al45tair/netifaces,al45tair/netifaces