HTTPSConnection.set_tunnel() regressed when it began calling .encode() directly on header names and values. Previously, formatting converted these values to strings, so values such as None and integers were accepted. Convert both names and values to strings before Latin-1 encoding, preserving that behavior and the existing CR/LF validation.

Fixes #159044

Tests

• ./python -m test test_httplib -m test_tunnel_headers_are_converted_to_strings — passed.
• ./python -m test test_httplib — passed (153 tests, 5 skipped).
• ./python Tools/patchcheck/patchcheck.py — passed.
• Full ./python -m test run: 449 test files passed, 96 skipped; failures were in test_os and test_pathlib mount/device assertions, test_socket VSOCK without a device, test_profiling (passed when rerun alone), and a shared-environment change reported by test_httpservers (passed alone).

AI tools assisted with investigation, implementation, and review. I reviewed the resulting changes.

• Issue: http.client.HTTPSConnection._tunnel raises AttributeError: 'NoneType' object has no attribute 'encode' when header value is None #159044