RFC: Enable Towncrier based change log support

[harshanarayana]

Enable usage of towncrier to generate and manage the release change logs automatically.

[andreymal]

Automatic changelogs are awful.

No automatics can make a good changelog, like this: https://docs.djangoproject.com/en/2.2/releases/2.0/

Or at least like this: https://websockets.readthedocs.io/en/stable/changelog.html

[harshanarayana]

@andreymal towncrier provides a mechanism whereby you can create a .bugfix .feature file in a specific directory with right markdown template and all it does is gather them and generate clean output with proper groupings.

This in no way is a replacement to manual detailed change logs but this is better than having to sit and go over the commit to extract meaningful information to generate Changelogs during the release time,.