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

_custom_build/backend.py

@@ -21,9 +21,7 @@ class _CustomBuildMetaBackend(backend_class):
                 sys.argv = sys.argv[:1] + ["build_ext"] + flags + sys.argv[1:]
         return super().run_setup(setup_script)
 
-    def build_wheel(
-        self, wheel_directory, config_settings=None, metadata_directory=None
-    ):
+    def build_wheel(self, wheel_directory, config_settings=None, metadata_directory=None):
         self.config_settings = config_settings
         return super().build_wheel(wheel_directory, config_settings, metadata_directory)