Import Self from typing_extensions not typing

This is needed for compatibility with older Python versions.
We should consider how useful that really is, or whether
we drop support for Python < 3.11

src/labthings_fastapi/descriptors/endpoint.py

@@ -8,11 +8,11 @@
     Literal,
     Mapping,
     Optional,
-    Self,
     Union,
     overload,
     TYPE_CHECKING,
 )
+from typing_extensions import Self  # 3.9, 3.10 compatibility
 from fastapi import FastAPI
 
 if TYPE_CHECKING: