requests: Match response header names case-insensitively.

[pablogventura]

Summary

• Fixes urequests: Detect specific header field names caselessly #523 (supersedes the 2022 urequests PR; the module is now requests).
• Match Transfer-Encoding and Location on the wire case-insensitively (RFC 9110 Sec. 5.1), same lowerl = l.lower() once-per-line pattern as urequests: Detect specific header field names caselessly #523 and aiohttp: Match response header names case-insensitively. #1146.
• Covers the requests half of aiohttp module case-insensitive headers #1126. aiohttp: Match response header names case-insensitively. #1146 stays aiohttp-only.

Test plan

• Unix test_requests.py, including lowercase transfer-encoding chunked and location redirect cases.
• __init__.mpy (mpy-cross, compiled as __init__.py): master 3160, this PR 3169 (+9).

[dpgeorge]

Could this line now be if lowerl.startswith(b"content-length:")? That saves a call to lower() and the associated memory overhead.

[pablogventura]

Thanks. Applied in e0d0462: if lowerl.startswith(b"content-length:") instead of k.lower() == "content-length", plus a lowercase content-length test.

__init__.mpy (mpy-cross, compiled as __init__.py): master 3160, previous on this branch 3169 (+9), e0d0462 still 3169 (+9). Same bytecode size; avoids the extra .lower() at runtime.

Unix test_requests.py passes.