Handle anonymous contributors

[CodingFeng101]

What changed

Contributor now handles anonymous contributor records returned by the contributors API when anon=true. Anonymous records keep their name, email, and contribution count without trying to parse missing user URL/avatar fields.

Why

This addresses #891. GitHub can return anonymous contributor records that do not include the full user payload. The previous parsing path always used _User._update_attributes(), so it raised IncompleteResponse from a missing field such as avatar_url before iteration could continue.

Validation

• python -m pytest tests\unit\test_users.py::TestContributor -q
• python -m pytest tests\unit\test_repos_repo.py::TestRepositoryIterator::test_contributors tests\unit\test_repos_repo.py::TestRepositoryIterator::test_contributors_with_anon -q
• python -m compileall -q src\github3\users.py tests\unit\test_users.py
• python -m black --check src\github3\users.py tests\unit\test_users.py
• python -m flake8 src\github3\users.py tests\unit\test_users.py
• git diff --check