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.

tests/transports/test_sslcontext.py

@@ -0,0 +1,11 @@
+import ssl
+
+from kasa.transports.sslcontext import get_ssl_context
+
+
+async def test_ssl_context_extends_python_defaults_with_legacy_suites() -> None:
+    """Some ciphers are added on top of Python's defaults for compatibility."""
+    default = {c["name"] for c in ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT).get_ciphers()}
+    enabled = {c["name"] for c in (await get_ssl_context()).get_ciphers()}
+    assert default <= enabled
+    assert {"AES128-GCM-SHA256", "AES256-SHA"} <= enabled