feat(config): support configuration via pyproject.toml

[TheKevJames]

Summary

Adds support for configuring coveralls-python via a [tool.coveralls] table in pyproject.toml, alongside the existing sources (CI env, COVERALLS_* env vars, .coveralls.yml, CLI flags).

TOML parsing is always available (stdlib tomllib on 3.11+, tomli on 3.10 — now an explicit dependency), so unlike the legacy YAML support it needs no extra install.

Behaviour

• New file source slots into the existing resolve() precedence stack: CI env → COVERALLS_* env → config file → CLI/kwarg overrides.
• Config files are never merged: the first file with settings wins, and the legacy .coveralls.yml takes precedence over pyproject.toml (matching the community norm used by coverage.py, mypy, pytest, and ruff, and preserving existing behaviour). A warning is emitted when both provide settings.
• [tool.coveralls] runs through the same alias/deprecation/unknown-key pipeline as .coveralls.yml.
• Malformed TOML surfaces as TOMLDecodeError; a non-table tool.coveralls raises TypeError; a missing table is silently ignored.

Refactor

coveralls/configuration.py grew past the 500-line limit, so it is split into a package with a strict one-directional import layering (repo → __init__ → {ci, environment, files} → helpers):

• helpers.py — Config, defaults, key canonicalization/filtering
• ci.py — CI service detection
• environment.py — COVERALLS_* env loading
• files.py — YAML/TOML file readers + first-found-wins selection
• __init__.py — resolve() entrypoint, re-exports Config

Each module owns its own logger (coveralls.configuration.<module>).

Docs

docs/usage/configuration.rst now presents pyproject.toml as the recommended approach and documents .coveralls.yml as legacy (still fully supported).

Tests

All 178 tests pass; pre-commit run -a is clean. New TOML coverage lives in tests/api/toml_config_test.py.