Add a changelog fragment for post-1.2 changes

[kurtmckee]

Python support

• Drop support for Python 3.6 and 3.7.
• Support Python 3.8+ and higher.

Changed

• Use orjson for speed.

Development

• Introduce a test suite.
• Add CI to test the project.
• Add CD to build and publish the project.
• Use Dependabot to maintain CI/CD action versions.
• Update pre-commit hooks.
• Use pre-commit.ci to maintain pre-commit hook versions,
  and to run hooks against incoming PRs.