adjust the release process to not require gen_version.py tweaks

adjust the release process to not require gen_version.py tweaks

This change has two primary motivations:

  1. Streamlining the release steps and reducing the opportunities for human
     error; and

  2. Improving clarity in the release artifacts.

Following this change, no edits are required to gen_version.py in order to cut
a new release. The Graphviz version – both inter-release and for releases
themselves – is derived exclusively from CHANGELOG.md. The effect of this is
expected to be:

  1. Cleaner release commits. No more regular churn of gen_version.py.

  2. More obvious CHANGELOG.md in the release artifacts. This file will no
     longer have a spurious “[Unreleased]” heading preceding the actual
     changelog for the release.