Implement Rendezqueue Python client

- Implement `RendezqueueClient` in `src/rendezqueue/client.py`.
- Add thread safety with `threading.Lock`.
- Add manual test script `test/manual_test.py`.
- Add unit tests `test/client_test.py`.
- Add CI workflow in `.github/workflows/ci.yaml`.
- Fix base64 padding issue.
- Set User-Agent to bypass Cloudflare.
- Add timeout to network requests.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: gendeux

src/rendezqueue/client.py

@@ -41,8 +41,8 @@ def stop(self):
         if self.is_stopped:
             return
         self.is_stopped = True
-        if self.poll_thread:
-            pass
+        # We don't join here because stop might be called from within a callback
+        # which might be running in the poll thread (if on_data is synchronous)
 
     def send(self, value):
         if isinstance(value, str):
@@ -93,7 +93,7 @@ def _poll(self):
             req = urllib.request.Request(self.url, data=data, headers=headers)
 
             try:
-                with urllib.request.urlopen(req) as response:
+                with urllib.request.urlopen(req, timeout=10) as response:
                     if response.status != 200:
                         text = response.read().decode('utf-8')
                         self.on_error(Exception(f"Server error: {response.status} {text}"))