Prep for jupyter releaser

[blink1073]

For now we will use the release from releaser-fork approach, until the bugs are worked out of the release-from-repo approach.

Used the following command and script for the changelog:

pandoc -f rst -t markdown -o CHANGELOG.md changelog.rst

import re
with open('CHANGELOG.md') as fid:
    data = fid.read()


patt = r'`(\d+)`({.interpreted-text\s+role="ghpull"}:?)'
matches = re.findall(patt, data)


for num, second in matches:
    data = data.replace(f"`{num}`{second}", f"[#{num}](https://github.com/jupyter/nbconvert/pull/{num})" )


with open("CHANGELOG.md", "w") as fid:
    fid.write(data)