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_async_write_large_payload.py

@@ -63,9 +63,7 @@ async def test_large_response_sequential():
         response = await cap.foo(i=size, j=False)
 
         # Verify the response has the correct length
-        assert (
-            len(response.x) == size
-        ), f"Size mismatch for {size}: expected {size}, got {len(response.x)}"
+        assert len(response.x) == size, f"Size mismatch for {size}: expected {size}, got {len(response.x)}"
 
         # Verify the pattern is correct (not corrupted)
         expected = "".join(chr(65 + (k % 26)) for k in range(size))
@@ -107,6 +105,4 @@ async def test_large_response_pipelined():
         assert len(response.x) == size, f"Size mismatch for {size}"
 
         expected = "".join(chr(65 + (k % 26)) for k in range(size))
-        assert (
-            response.x == expected
-        ), f"Data corruption detected for {size} bytes payload!"
+        assert response.x == expected, f"Data corruption detected for {size} bytes payload!"