[Twisted-Python] overview: new review queue venue

[meejah]

Personally, I find closing PRs that aren't going to be merged "soon"
mostly-beneficial. Even if it *might* be perceived as "hostile" by some
contributers, a simple explanation should suffice. (And, if simply
closing a PR with a nice note explaining, "please re-open when X is
fixed/changed" scares away a potential contributer I have my doubts as
to whether they would fix X if you *didn't* close it...)

There's nothing worse than trolling through open PRs only to find that
the last comment is "fix up X, Y, and Z and we'll merge" because then
you have to (re-)figure out if X, Y and Z have been done etc. On the
flip side, it's nice to know if your PR has problems or not.

The other plus of closing is that it's way more obvious when the PR is
once again considered ready for merging (even without 'completely baked'
workflows like Twisted's) and keeps the "open PRs" list (hopefully!)
shorter.

-- 
meejah