gh-157951: Don't read a chunked body for 1xx, 204 and 304 responses in http.client

[DrVelvetFog]

HTTPResponse.begin() already sets length = 0 for 1xx, 204, 304 and HEAD, but left chunked set when the response had Transfer-Encoding: chunked, so read() blocked in _read_chunked(). This clears chunked in the same place. HEAD was already handled by the check in read(), and that check stays.

Test: test_bodiless_status_with_chunked_sync sends a 204 and a 304 with Transfer-Encoding: chunked followed by another response, and checks that read() returns b"" and leaves the next response unread. It fails without the change.

• Issue: http.client hangs reading a 304 or 204 response that has Transfer-Encoding: chunked #157951

[python-cla-bot]

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[aisk]

I applied the test locally and confirmed it fails, then applied the fix and it passes. LGTM.