Prepare Release workflow and towncrier changelog (as in anyplotlib)

[CSSFrancis]

Brings deapi's release process in line with anyplotlib and de-twin: a Prepare Release workflow plus a towncrier changelog.

How a release works after this

1. Actions → Prepare Release → Run workflow, then pick the bump:
   • pre-release: 5.3b6 → 5.3.0b7
   • finalize: 5.3b6 → 5.3.0
   • minor, bugfix, major, with an optional beta flag
2. The workflow bumps version in pyproject.toml, builds CHANGES.rst from the fragments in upcoming_changes/, and opens a Release v… PR. Review it and merge.
3. Create the GitHub release by hand, as today: tag v<version> on main, and tick "pre-release" for betas. Publishing it runs the existing publish.yaml, which uploads to PyPI.

What's included

• .github/workflows/prepare_release.yml: anyplotlib's workflow, with these changes:
  • It parses deapi's two-part versions (5.3b6).
  • It leaves out the docs version-switcher steps, since deapi's docs aren't versioned.
  • Its PR text points to the manual GitHub release instead of a tag push.
• [tool.towncrier] in pyproject.toml: the same fragment types as anyplotlib (api_change, new_feature, bugfix, deprecation, removal, doc, maintenance). Entries link to deapi PRs.
• upcoming_changes/README.rst: how to name and write fragments, adapted from anyplotlib's.
• CHANGES.rst: a .. towncrier release notes start marker above the existing entries. Older entries are unchanged.

Choices to review

• Version format. Versions become major.minor.patch, so 5.3b6 is followed by 5.3.0b7. That's the same version as 5.3b7 under PEP 440, so pip ordering doesn't change. Tags follow as v5.3.0b7, replacing the v5.3beta7 style.
• deapi/version.py isn't bumped. Its version is the client/protocol version that the client sends to DE-Server, and versionInfo parses it as integers. The workflow only changes the package version in pyproject.toml.
• publish.yaml is unchanged. PyPI's trusted publisher is tied to that workflow's filename, so renaming or rewriting it would need the PyPI settings changed too.
• Release PRs don't start CI automatically. They're opened with GITHUB_TOKEN, as in anyplotlib, so push an empty commit or close and reopen the PR to run it.

Tested locally

• The workflow's version logic, run against 5.3b6, 5.3.0b7 and 5.3.0 with every bump, gives the versions above. It refuses bumps that would skip a pending 5.3.0.
• towncrier build --yes, run on a copy with this PR's fragment, writes a new 5.3.0b7 (date) section under the marker and deletes the fragment.
• The workflow file parses as valid YAML.
• I haven't run the workflow on GitHub itself; the first real run will be the next release.

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