tools: add https server script

In order to test on Android, we must serve the local build directory
with https, otherwise Web Bluetooth doesn't work.

tools/serve.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+
+"""
+Serve local build with https for testing.
+
+This is useful, e.g. for Android (WebBluetooth doesn't work without https).
+
+Usage:
+
+    yarn build
+    ./tools/serve.py
+
+Browse to https://localhost:8443 on the local machine or
+https://<name-or-address>:8443 on a remote device.
+
+The browser will complain about the self-signed certificate, but this can be
+bypassed (click "Advanced" in browser).
+"""
+
+from http.server import HTTPServer, SimpleHTTPRequestHandler
+from pathlib import Path
+import os
+import ssl
+
+THIS_DIR = Path(__file__).parent
+TEST_CERT = (THIS_DIR / 'test.pem').absolute()
+BUILD_DIR = (THIS_DIR / '..' / 'build').absolute()
+
+# HTTPServer serves the current directory.
+os.chdir(BUILD_DIR)
+
+httpd = HTTPServer(('', 8443), SimpleHTTPRequestHandler)
+httpd.socket = ssl.wrap_socket(httpd.socket, certfile=TEST_CERT, server_side=True)
+httpd.serve_forever()