nipanel: Extract typing backports into a submodule (#14)

* _typing.py

* only use _typing.py for types that don't exist in typing

* better ParamSpec

---------

Co-authored-by: Mike Prosser <Mike.Prosser@emerson.com>

Author: mikeprosserni

src/nipanel/_panel_client.py

@@ -4,21 +4,18 @@
 
 import logging
 import threading
-from typing import TYPE_CHECKING, Callable, TypeVar
+from typing import TypeVar, Callable
 
 import grpc
 from ni.pythonpanel.v1.python_panel_service_pb2 import OpenPanelRequest
 from ni.pythonpanel.v1.python_panel_service_pb2_grpc import PythonPanelServiceStub
 from ni_measurement_plugin_sdk_service.discovery import DiscoveryClient
 from ni_measurement_plugin_sdk_service.grpc.channelpool import GrpcChannelPool
 
-_T = TypeVar("_T")
-
-if TYPE_CHECKING:
-    from typing_extensions import ParamSpec
-
-    _P = ParamSpec("_P")
+from nipanel._typing import ParamSpec
 
+_P = ParamSpec("_P")
+_T = TypeVar("_T")
 
 _logger = logging.getLogger(__name__)
 

src/nipanel/_typing.py

@@ -0,0 +1,25 @@
+"""Single source for typing backports to avoid depending on typing_extensions at run time."""
+
+from __future__ import annotations
+
+import sys
+from typing import TYPE_CHECKING
+
+if sys.version_info >= (3, 11):
+    from typing import Self
+elif TYPE_CHECKING:
+    from typing_extensions import Self
+else:
+    Self = None
+
+if sys.version_info >= (3, 10):
+    from typing import ParamSpec
+elif TYPE_CHECKING:
+    from typing_extensions import ParamSpec
+else:
+    from typing import TypeVar as ParamSpec
+
+__all__ = [
+    "Self",
+    "ParamSpec",
+]