Request for comment: new changelog process

[Aaron Meurer]

There have been many discussions for some time about the poor quality
of our changelogs. Basically, the issue is that almost no one updates
the release notes (currently on the wiki
https://github.com/sympy/sympy/wiki/Release-Notes) when a PR is
merged, so it is generally left to me to do at release time. At this
point, there are hundreds of merged pull requests, so the whole
process ends up being ad hoc and incomplete.

A suggested new process has been implemented here:
https://github.com/sympy/sympy/pull/13584

With this change, every pull request should have some text in the pull
request description

## This PR changes...
M: This is a major change
M : This is another major change...
m: Tiny, tiny... change
b :Something has been deprecated!
n : New feature! Hooray!
<!-- [CHANGELOG END] -->

There is also some text in the pull request template to tell people
about this. If no release note are added, the tests will fail, unless
the text [skip changelog] is there.

When the pull request is merged, a script automatically adds the
changes to the respective release notes wiki page (see for instance
https://github.com/sympy/sympy/wiki/Release-Notes-for-1.1.2).

The advantage of this is that the release notes can easily be modified
by the person who made the PR, as well as by anyone with push access
(anyone with push access can edit the PR description).

Since this will affect development process of everyone making PRs to
SymPy, comments on https://github.com/sympy/sympy/pull/13584 would be
appreciated before it is merged.

Aaron Meurer