BUG: use compression=None (again) to avoid inferring compression
`read_fwf` was using `compression=None` to infer compression (in addition to `compression="infer"`). This is undocumented but was enforced in a test.

In #37639, I thought this was the expected behavior for all functions. That as wrong, `read_csv` explicitly states that `compression=None` does not infer any compression.
