The 'attrs' library is now available within Mercurial -- a better namedtuple

[Siddharth Agarwal]

Hi everyone --

The attrs library is now available to use within Mercurial as 
'mercurial.thirdparty.attrs'. attrs is basically a better version of 
collections.namedtuple with none of the weird semantics (e.g. you can't 
compare a tuple and an attr) and no runtime performance cost (especially 
with slots turned on).

The documentation for attrs is available at [1]. See e51c8ffa1ffa [2] 
for an example of how to use attrs.

Please feel free to send cleanup patches to move existing tuples or 
namedtuples to use attrs.

[1] http://www.attrs.org/en/stable/
[2] https://www.mercurial-scm.org/repo/hg/rev/e51c8ffa1ffa