Rename NEWS fragment

This build just failed
https://github.com/pypa/pipenv/runs/7853944342?check_suite_focus=true
with the following error:

    NEWS fragment files must be named *.(feature|behavior|bugfix|vendor|doc|trivial|removal|process).rst

    news/5261.bugfix

    pre-commit hook(s) made changes.
    If you are seeing this message in CI, reproduce locally with: `pre-commit run --all-files`.

I created this file by following the PR template instructions, which
don't say anything about the `.rst` part of the filename. I'm fixing up
those instructions while I'm in here as well.

.github/PULL_REQUEST_TEMPLATE.md

@@ -19,7 +19,7 @@ How does this pull request fix your problem? Did you consider any alternatives?
 ### The checklist
 
 * [ ] Associated issue
-* [ ] A news fragment in the `news/` directory to describe this fix with the extension `.bugfix`, `.feature`, `.behavior`, `.doc`. `.vendor`. or `.trivial` (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.
+* [ ] A news fragment in the `news/` directory to describe this fix with the extension `.bugfix.rst`, `.feature.rst`, `.behavior.rst`, `.doc.rst`. `.vendor.rst`. or `.trivial.rst` (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.
 
 <!--
 ### If this is a patch to the `vendor` directory...