fix: avoid deprecated aiohttp basic auth API

[Pybsama]

Fixes

Fixes #931

Replace the deprecated aiohttp BasicAuth / auth= request path with an Authorization header in AsyncTwilioHttpClient.

The implementation feature-detects encode_basic_auth so the declared aiohttp>=3.8.4 range remains supported, explicitly preserves the historical Latin-1 wire encoding, rejects conflicting Authorization headers case-insensitively, and does not mutate caller-provided headers. The network call no longer passes the auth keyword, which also avoids the second aiohttp 3.14 deprecation and remains compatible with its planned removal in aiohttp 4.

Validation

• Python 3.9.6 + aiohttp 3.8.4: 17 focused tests passed
• Python 3.14.6 + aiohttp 3.14.3: 17 focused tests passed
• Real local aiohttp request with aiohttp deprecations treated as errors: passed
• Full non-cluster suite: 639 tests passed
• make analysis: passed
• Changed-file Black, Autoflake, and git diff --check: passed

make test reaches the repository-wide Black check and stops on the pre-existing formatting of tests/unit/rest/test_client.py. That file is unchanged from main; the complete pytest suite and both Flake8 commands were run separately as listed above.

Checklist

• I acknowledge that all my contributions will be made under the project license
• I have made a material change to the repo
• I have read the Contribution Guidelines and this PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove the fix is effective
• No user-facing documentation is needed because the public API and authentication behavior are unchanged
• I have documented the compatibility helper added by this change