Have create_release_notes.py get the old version from ReleaseNotes.md (#3275)

This makes it dependent on ReleaseNotes.md, which makes sense. In
addition, it now can only add one release at a time, which seems fine.

I also added some tests, because I wanted to make sure it works without
having to fetch commit history and whatnot.
I still had to do some validation by running the command line, and found
some bugs that way. I resolved those issues, and made it safer by adding
type hints.

I also, obviously, did not add any integration into prb for the tests,
but that may be nice if we right more, they currently take less than a
second.

Author: ScottDugas

.github/workflows/release.yml

@@ -165,7 +165,7 @@ jobs:
           --repository ${{ github.repository }} \
           --commit \
           --mixed-mode-results mixed-mode-results.md \
-          ${{ steps.get_old_version.outputs.version }} ${{ steps.get_new_version.outputs.version }}
+          --version ${{ steps.get_new_version.outputs.version }}
         env:
           GH_TOKEN: ${{ github.token }}
       # We move the tag to after the release notes are updated so that later steps (i.e. sphinx) will pick up the udpated