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_capability_context.py

@@ -212,9 +212,7 @@ class TailCaller(capability.TestTailCaller.Server):
     async def foo_context(self, context):
         self.count += 1
 
-        tail = context.params.callee.foo_request(
-            i=context.params.i, t="from TailCaller"
-        )
+        tail = context.params.callee.foo_request(i=context.params.i, t="from TailCaller")
         await context.tail_call(tail)