Replace black and flake8 with ruff for linting and formatting

- Remove .flake8; add [tool.ruff] and [tool.ruff.format] config in pyproject.toml
  (line-length 120, excludes, ignore list, per-file-ignores, mccabe complexity)
- Update GitHub workflow lint job to run `ruff check .` and `ruff format --check .`
- Swap black and flake8 for ruff in requirements.txt and Pipfile
- Change capnp/__init__.py to ruff-style noqa comment
- Move max-complexity into [tool.ruff.lint.mccabe], lint options into [tool.ruff.lint]
- Add per-file-ignores for capnp/__init__.py (F401, F403, F405), remove inline noqa
- Run ruff format across codebase (24 files) for consistent style

test/test_serialization.py

@@ -114,9 +114,7 @@ def test_roundtrip_bytes_packed(all_types):
 
 
 @contextmanager
-def _warnings(
-    expected_count=2, expected_text="This message has already been written once."
-):
+def _warnings(expected_count=2, expected_text="This message has already been written once."):
     with warnings.catch_warnings(record=True) as w:
         yield