fix(client): encode and scope HTTP Basic credentials

[jcardozo-eth]

Closes #41

Summary

Configuring Client with a username and password raised AttributeError before it could make a request because it called a removed Base64 function.

This pull request:

• Builds HTTP Basic credentials (RFC 7617) from UTF-8 bytes with the supported Base64 API.
• Sends the resulting Authorization header on both POST and forced-GET requests.
• Adds the header with Request.add_unredirected_header(), so urllib does not copy credentials to automatically redirected requests.

Test plan

• uv run pytest src/oaipmh/tests/test_client.py -k "http_basic_authentication or request_without_credentials_omits_authorization" exercises authenticated POST, forced GET, text credential encoding, redirect confinement, and unauthenticated requests.
• uv run pytest runs the complete test suite; all 83 tests pass locally.
• CI runs the complete suite in the repository's Python 3.10 through 3.14 Nix environments.

Checklist

• Tests green locally (uv run pytest)
• Python 3.10 through 3.14 test matrix green in CI