Add support for async callables in PythonOperator

[dabla]

This PR is related to the discussion I started on the devlist and which allows you to natively execute async code on PythonOperators.

There is also an AIP for this: https://cwiki.apache.org/confluence/display/AIRFLOW/%5BWIP%5D+AIP-98%3A+Rethinking+deferrable+operators%2C+async+hooks+and+performance+in+Airflow+3

Below an example which show you how it can be used with async hooks:

@task(show_return_value_in_logs=False)
async def load_xml_files(files):
    import asyncio
    from io import BytesIO
    from more_itertools import chunked
    from os import cpu_count
    from tenacity import retry, stop_after_attempt, wait_fixed

    from airflow.providers.sftp.hooks.sftp import SFTPClientPool

    print("number of files:", len(files))

    async with SFTPClientPool(sftp_conn_id=sftp_conn, pool_size=cpu_count()) as pool:
        @retry(stop=stop_after_attempt(3), wait=wait_fixed(5))
        async def download_file(file):
            async with pool.get_sftp_client() as sftp:
                print("downloading:", file)
                buffer = BytesIO()
                async with sftp.open(file, encoding=xml_encoding) as remote_file:
                    data = await remote_file.read()
                    buffer.write(data.encode(xml_encoding))
                    buffer.seek(0)
                return buffer

        for batch in chunked(files, cpu_count() * 2):
            tasks = [asyncio.create_task(download_file(f)) for f in batch]

            # Wait for this batch to finish before starting the next
            for task in asyncio.as_completed(tasks):
                result = await task
		 # Do something with result or accumulate it and return it as an XCom

This PR will fix additional remarks made by @kaxil on the original PR which has been reverted.

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In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[kaxil]

Almost there, once newsfragment is added we are good

[dabla]

Almost there, once newsfragment is added we are good

And the newfragment ;-)

[dabla]

@kaxil If newsfragment is ok for you then I think it's finally ready to merge.

[kaxil]

Static check is failing, worth looking at https://github.com/apache/airflow/blob/main/airflow-core/newsfragments/54505.significant.rst or other examples