use asyncio.run in Template.render

[graingert]

• fixes #1951

Checklist:

• [ ] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [ ] Add .. versionchanged:: entries in any relevant code docs.
• [ ] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.