Allows iterating over ChainableUndefined when using render_async.

Added __aiter__ method to Undefined which ChainableUndefined inherits from.

• fixes TypeError: 'async for' requires an object with __aiter__ method, got ChainableUndefined #1293

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• 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.
• Run pytest and tox, no tests failed.