SSCursor: Fix connection closed check

[FichteFoll]

What do these changes do?

Since the connection closes itself on an asyncio.CancelledError, it cannot be used afterwards. SSCursor.close attempts to clear some unfinished state first before closing using Connection._finish_unbuffered_query, but that call fails if the connection has been closed already and SSCursor itself does not properly check that.

Are there changes in behavior for the user?

Cancellations now do not raise an unhandled exception anymore when using the connection and the SSCursor in a context manager.

Following is an example traceback where both the connection and the SSCursor are used in an AsyncExitStack and the task was cancelled:

        async with contextlib.AsyncExitStack() as exit_stack:
            conn: aiomysql.Connection = await exit_stack.enter_async_context(get_connection())
            # Use an unbuffered cursor so we don't load the entire table into memory
            cursor: aiomysql.Cursor = await exit_stack.enter_async_context(conn.cursor(SSCursor))
            ...

Traceback (most recent call last):
  File "code/export/__init__.py", line 177, in _sql_to_csv
    while row := await cursor.fetchone():
                 ^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/cursors.py", line 629, in fetchone
    row = await self._read_next()
          ^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/cursors.py", line 622, in _read_next
    row = await self._result._read_rowdata_packet_unbuffered()
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/connection.py", line 1239, in _read_rowdata_packet_unbuffered
    packet = await self.connection._read_packet()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/connection.py", line 635, in _read_packet
    recv_data = await self._read_bytes(bytes_to_read)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/connection.py", line 657, in _read_bytes
    data = await self._reader.readexactly(num_bytes)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/streams.py", line 752, in readexactly
    await self._wait_for_data('readexactly')
  File "/usr/lib/python3.12/asyncio/streams.py", line 545, in _wait_for_data
    await self._waiter
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "code/export/__init__.py", line 69, in _wrap_with_log_msg
    result = await coro
             ^^^^^^^^^^
  File "code/export/__init__.py", line 85, in export_provider
    await self._export_table(
  File "code/export/__init__.py", line 154, in _export_table
    await self._sql_to_csv(table, csv_path, columns, joins)
  File "code/export/__init__.py", line 167, in _sql_to_csv
    async with contextlib.AsyncExitStack() as exit_stack:
  File "/usr/lib/python3.12/contextlib.py", line 754, in __aexit__
    raise exc_details[1]
  File "/usr/lib/python3.12/contextlib.py", line 737, in __aexit__
    cb_suppress = await cb(*exc_details)
                  ^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/utils.py", line 78, in __aexit__
    await self._obj.close()
  File "venv/lib/python3.12/site-packages/aiomysql/cursors.py", line 605, in close
    await self._result._finish_unbuffered_query()
  File "venv/lib/python3.12/site-packages/aiomysql/connection.py", line 1258, in _finish_unbuffered_query
    packet = await self.connection._read_packet()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/connection.py", line 609, in _read_packet
    packet_header = await self._read_bytes(4)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/aiomysql/connection.py", line 657, in _read_bytes
    data = await self._reader.readexactly(num_bytes)
                 ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'readexactly'

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[FichteFoll]

Tests are passing but submitting coverage data failed.
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