Set up scriv for changelog management

[mdickinson]

Let's use scriv for changelog management. This is very similar to the setup that we have in TraitsUI and apptools, except that it seems better to use scriv than grow our own tool.

scriv is relatively unopinionated, flexible and easy to use; it simply lets you assemble changelog snippets in separate files under changelog.d until you're ready to collect those snippets together to assemble a changelog entry for a release. It seems like a good fit for Traits.

We're currently lacking a good place to record the information that we're expecting to use scriv for changelog management. See #1803.

xref: https://scriv.readthedocs.io/en/1.5.1/