Motivation:

Previously, using async iteration with `psycopg` (`async for`) would
throw an exception.

Example:

```
def connect_to_db():
    # db connection logic...

async def testfunc():
    connectstr = get_conn_str()
    async with AsyncConnectionPool(
        conninfo=connectstr, min_size=4, max_size=30
    ) as async_pool:
        async with async_pool.connection() as conn:
            async with conn.cursor() as cur:
                await cur.execute(
                    sql.SQL(
                        """
                        SELECT
                            *
                        FROM table1
                        LIMIT 5
                    """
                    )
                )
                async for row in cur:
                    print(row)
```

Running `ddtrace-run python test.py` would return `TypeError: 'async
for' requires an object with __aiter__ method, got
Psycopg3TracedAsyncCursor`.

This fix implements the `__aiter__` method for the `TracedAsyncCursor`
parent class, and has `Psycopg3TracedAsyncCursor` inherit from it.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: Munir Abdinur <[email protected]>
(cherry picked from commit d51fa8d)