[python-committers] Small change to tracker 'resolution'

[R. David Murray]

In a discussion at the sprints today Nick observed that 'invalid' was a
resolution that carried a rather negative subtext ("your bug report was
invalid", ie: "you made a mistake") , and that Red Hat used 'not a bug',
which we all agreed was much more descriptive of the actual resolution.

So, I've changed the text associated with the 'invalid' resolution
to read 'not a bug' instead.

--David