Consolidate SSL ciphers and use them in SslTransport (#1798)

- Add `kasa/transports/sslcontext.py` with one shared SSL context for
the https transports to avoid copies across transports.
- Use it in `SslTransport`, and convert KLAP and Linkie also to use it.
- Extend Python's default ciphers with the legacy suites instead of
replacing accepted ciphers for future compatibility.

kasa/transports/sslcontext.py

@@ -0,0 +1,40 @@
+"""Shared SSL context for the transports talking https to local devices.
+
+Not security sensitive: certificates are not verified.
+The extra ciphers exist only for compatibility,
+as Python's defaults lack suites older firmware needs.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import ssl
+from functools import cache
+
+LEGACY_CIPHERS = [
+    "AES256-GCM-SHA384",
+    "AES256-SHA256",
+    "AES128-GCM-SHA256",
+    "AES128-SHA256",
+    "AES256-SHA",
+    "AES128-SHA",
+    "ECDHE-RSA-AES128-SHA",
+    "ECDHE-RSA-AES256-SHA",
+]
+
+
+@cache
+def _create_ssl_context() -> ssl.SSLContext:
+    """Return the cached context, blocking on first use."""
+    context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+    # Append to the defaults so suites newer firmware needs are not dropped.
+    default = [cipher["name"] for cipher in context.get_ciphers()]
+    context.set_ciphers(":".join([*default, *LEGACY_CIPHERS]))
+    context.check_hostname = False
+    context.verify_mode = ssl.CERT_NONE
+    return context
+
+
+async def get_ssl_context() -> ssl.SSLContext:
+    """Return the cached context, created in an executor as that blocks."""
+    return await asyncio.get_running_loop().run_in_executor(None, _create_ssl_context)