Don't import aiohttp when importing datasets

[kuldeeepy]

import datasets imports aiohttp just to reference aiohttp.ClientError in the two retry except clauses in file_utils.py. That's about 50 ms of a ~470 ms import (python -X importtime -c "import datasets"), and nothing else imports aiohttp at that point.

This looks the error type up when the except clause runs, and only if aiohttp is already in sys.modules: if it was never imported, it can't have raised. This follows the "x" in sys.modules checks used elsewhere in the library.

Tests: test_import_datasets_does_not_import_aiohttp fails on main and passes here; test_read_with_retries_retries_aiohttp_client_error checks aiohttp errors are still retried. tests/test_file_utils.py passes and make quality is clean.

AI assistance (Claude) was used in preparing this change; I reviewed it and ran the tests locally.

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