Add trailing comma for single-element import-from groups (#6583)
## Summary
Unlike other statements, Black always adds a trailing comma if an
import-from statement breaks with a single import member. I believe this
is for compatibility with isort -- see
https://github.com/psf/black/commit/09f5ee3a19f4274bb848324867bd8e68724cf851,
https://github.com/psf/black/issues/127, or
https://github.com/psf/black/blob/66648c528a95553c1f822ece394ac98784baee47/src/black/linegen.py#L1452
for the current version.
## Test Plan
`cargo test`, notice that a big chunk of the compatibility suite is
removed.
Before:
| project | similarity index |
|--------------|------------------|
| cpython | 0.75472 |
| django | 0.99804 |
| transformers | 0.99618 |
| twine | 0.99876 |
| typeshed | 0.74233 |
| warehouse | 0.99601 |
| zulip | 0.99727 |
After:
| project | similarity index |
|--------------|------------------|
| cpython | 0.75472 |
| django | 0.99804 |
| transformers | 0.99618 |
| twine | 0.99876 |
| typeshed | 0.74260 |
| warehouse | 0.99601 |
| zulip | 0.99727 |
