Avoid the unnecessary import of typing_extensions

Relevant links:

- https://typing-extensions.readthedocs.io/en/latest/index.html#typing_extensions.get_args (since Python 3.8)
- https://typing-extensions.readthedocs.io/en/latest/index.html#typing_extensions.get_origin (since Python 3.8)
- https://typing-extensions.readthedocs.io/en/latest/index.html#typing_extensions.get_type_hints (since Python 3.?)
- https://typing-extensions.readthedocs.io/en/latest/index.html#typing_extensions.Annotated (since Python 3.9)
- https://typing-extensions.readthedocs.io/en/latest/index.html#typing_extensions.Literal (since Python 3.8)

typer/_typing.py

@@ -13,7 +13,10 @@
     Union,
 )
 
-from typing_extensions import Literal, get_args, get_origin
+if sys.version_info >= (3, 8):
+    from typing import Literal, get_args, get_origin
+else:
+    from typing_extensions import Literal, get_args, get_origin
 
 if sys.version_info < (3, 10):
 

typer/main.py

@@ -15,7 +15,10 @@
 from uuid import UUID
 
 import click
-from typing_extensions import get_args, get_origin
+if sys.version_info >= (3, 8):
+    from typing import get_args, get_origin
+else:
+    from typing_extensions import get_args, get_origin
 
 from ._typing import is_union
 from .completion import get_completion_inspect_parameters

typer/utils.py

@@ -3,7 +3,15 @@
 from copy import copy
 from typing import Any, Callable, Dict, List, Tuple, Type, cast
 
-from typing_extensions import Annotated, get_args, get_origin, get_type_hints
+if sys.version_info >= (3, 9):
+    from typing import Annotated
+else:
+    from typing_extensions import Annotated
+
+if sys.version_info >= (3, 8):
+    from typing import get_args, get_origin, get_type_hints
+else:
+    from typing_extensions import get_args, get_origin, get_type_hints
 
 from .models import ArgumentInfo, OptionInfo, ParameterInfo, ParamMeta