Allow triagers to close PRs if the PR is of very low quality (#828)

As discussed on the CPython core-dev discord channel, there are several issues of the current policy that states that triagers should never close PRS:
- Github's setup allows members of the triage team to close PRs, regardless of the policy stated in the devguide.
- This policy isn't consistently followed by all members of the triage team. Some triagers occasionally close PRs (I have closed one or two myself in situations where it seemed abundantly clear that there was no prospect of the PR being merged).
- This policy isn't really enforced in any way if triagers do close PRs. This lack of enforcement seems to be an implicit endorsement of the idea that it is okay for triagers to close PRs in some circumstances.
- It feels silly to have to bother a core dev with a request for a PR to be closed if it's self-evident that it should be closed (e.g. if the PR makes changes to a deprecated module, makes solely cosmetic changes to a module, or simply has a 0% chance of ever being merged)
- Changing this policy will help towards the goal of reducing the CPython PR backlog

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

Author: AlexWaygood

triaging.rst

@@ -35,17 +35,31 @@ Responsibilities include:
     - Good first issue
     - Other categorizations
 
-As triagers gain experience, they may have some intuition of when a PR should
-be closed. Triagers can recommend closing a PR, but the final decision must be
-made by a core developer. By having triagers and core developers work together,
+Although triagers have the power to close PRs, they should generally not do so
+without first consulting a core developer. By having triagers and core developers work together,
 the author receives a careful consideration of their PR. This encourages future
 contributions, regardless of whether their PR is accepted or closed.
 
-Triagers can make use of the ``invalid`` and ``stale`` labels to suggest that a
+Nonetheless, triagers should feel free to close a PR if they judge that the
+chance of the PR being merged would be exceedingly low, even if substantial
+revisions were made to the PR. This includes (but is not limited to) the
+following:
+
+* PRs proposing solely cosmetic changes
+* PRs proposing changes to deprecated modules
+* PRs that are no longer relevant. This includes:
+    - PRs proposing fixes for bugs that can no longer be reproduced
+    - PRs proposing changes that have been rejected by Python core developers
+      elsewhere (e.g. in an issue or a PEP rejection notice)
+
+If a triager has any doubt about whether to close a PR, they should consult a core
+developer before taking any action.
+
+Triagers can also make use of the ``invalid`` and ``stale`` labels to suggest that a
 PR may be suitable for closure. For more information, see the
 :ref:`GitHub PR labels <github-pr-labels>` section.
 
-It is also of paramount importance to treat every contributor to the Python
+Note that it is of paramount importance to treat every contributor to the Python
 project kindly and with respect. Regardless of whether they're entirely new
 or a veteran core developer, they're actively choosing to voluntarily donate their
 time towards the improvement of Python. As is the case with any member of