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

examples/async_server.py

@@ -28,9 +28,7 @@ async def new_connection(stream):
 
 
 def parse_args():
-    parser = argparse.ArgumentParser(
-        usage="""Runs the server bound to the given address/port ADDRESS. """
-    )
+    parser = argparse.ArgumentParser(usage="""Runs the server bound to the given address/port ADDRESS. """)
 
     parser.add_argument("address", help="ADDRESS:PORT")