avoid ResourceWarning in `DataBody.__aiter__`

[graingert]

Avoid a ResourceWarning by avoiding an async generator

• fixes #301

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.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.

[graingert]

tests are tricky for this, as it only causes the warning on quart-trio

[pgjones]

Thanks