Towncrier support and plugin

[mauritsvanrees]

For some Plone packages we want to start using towncrier to handle updating CHANGES.rst, just like for example pip does. For explanation, see https://github.com/plone/plone.releaser/issues/17. (plone.releaser is a wrapper around zest.releaser that we use when creating releases for Plone packages.)

This may need to be done in a separate small plugin. But it needs a few changes in zest.releaser. Mainly: when towncrier updates the CHANGES.rst, zest.releaser should not touch that file at all.

So I made two commits:

• one with a few minor changes in zest.releaser: 39b465ed642b7420b0bfe5e0d79b567978b5b605
• one with the hooks for calling towncrier: acd9b97a02fb681c0a0d72aaa253a52516aa8a90

The second part may need to be moved to a separate plugin, and I am happy to do so. It could be okay to include it in zest.releaser, and it at least makes it easier to try out this PR.

I have used this code to make release 4.0.1 of collective.recipe.backup. Most important are these commits:

• prerelease
• postrelease

So:

• review welcome on the first commit with the minor zest.releaser changes
• feedback welcome on the towncrier hooks and whether that should be a separate plugin

[mauritsvanrees]

Thanks for the feedback. I will create a plugin for this, probably in the Plone collective. I will close this PR and create a new one for clarity, where I will only do the update_history stuff, and not the update_version.