Code quality improvements and test fixes (#592)

## Summary
Comprehensive post-v3.0.0 modernization: new features, bug fixes,
dependency cleanup, docs, and test coverage.

**New features:**
- **HTTP method filtering** — `@api.route("/data", methods=["GET"])`
- **Lifespan context manager** — modern async startup/shutdown
- **`api.exception_handler()`** — custom error handling per exception
type
- **`api.graphql()`** — one-liner GraphQL setup
- **`resp.file()`** — serve files from disk with auto content-type
- **before_request short-circuit** — set status code to skip route
handler
- **`req.path_params`** / **`req.client`** / **`req.is_json`** — new
request properties
- **`uuid`** and **`path`** route convertors
- **PEP 561 `py.typed`** marker

**Bug fixes:**
- Fix multipart parser losing headers
- Fix `url_for()` with typed params (`{id:int}`)
- Fix `resp.body` encoding crash on bytes content
- Fix Python 3.9 type syntax (`from __future__ import annotations`)
- Fix broken session test and no-op file upload test
- Fix helloworld example 404 on root path

**Dependencies:**
- Flattened — `pip install responder` gets everything
- Core: just starlette + uvicorn (down from 10 deps)

**Docs & README:**
- All new features documented in tour
- Modernized README features list
- Deployment guide: Docker, cloud, uvicorn
- Removed Pipenv, extras, stale references throughout

**Tests & quality:**
- 117 tests (up from 92), 91% coverage, 0 warnings
- CaseInsensitiveDict, GraphQL edge cases, staticfiles tests
- Ruff clean, all `tmpdir` → `tmp_path`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/helloworld.py

@@ -5,6 +5,11 @@ import responder
 api = responder.API()
 
 
+@api.route("/")
+async def index(req, resp):
+    resp.text = "hello, world!"
+
+
 @api.route("/{greeting}")
 async def greet_world(req, resp, *, greeting):
     resp.text = f"{greeting}, world!"

examples/lifespan.py

@@ -0,0 +1,26 @@
+# Example showing the lifespan context manager pattern.
+# https://pypi.org/project/responder/
+from contextlib import asynccontextmanager
+
+import responder
+
+
+@asynccontextmanager
+async def lifespan(app):
+    # Startup: initialize resources
+    print("Starting up...")
+    yield
+    # Shutdown: clean up resources
+    print("Shutting down...")
+
+
+api = responder.API(lifespan=lifespan)
+
+
+@api.route("/{greeting}")
+async def greet_world(req, resp, *, greeting):
+    resp.text = f"{greeting}, world!"
+
+
+if __name__ == "__main__":
+    api.run()