Make typing-extensions optional

Fixes pydataGH-5495

Type checking may be a little worse if typing-extensions are not
installed, but I don't think it's worth the trouble of adding another
hard dependency just for one use for TypeGuard.

setup.cfg

@@ -67,6 +67,7 @@ classifiers =
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
     Topic :: Scientific/Engineering
 
 [options]
@@ -78,7 +79,6 @@ install_requires =
     numpy >= 1.17
     pandas >= 1.0
     setuptools >= 40.4  # For pkg_resources
-    typing-extensions >= 3.10  # Backported type hints
 
 [options.extras_require]
 io =

xarray/core/utils.py

@@ -32,10 +32,14 @@
 import numpy as np
 import pandas as pd
 
-if sys.version_info >= (3, 10):
-    from typing import TypeGuard
-else:
-    from typing_extensions import TypeGuard
+try:
+    if sys.version_info >= (3, 10):
+        from typing import TypeGuard
+    else:
+        from typing_extensions import TypeGuard
+    TypeGuardHashable = TypeGuard[Hashable]
+except ImportError:
+    TypeGuardHashable = bool
 
 
 K = TypeVar("K")
@@ -297,7 +301,7 @@ def either_dict_or_kwargs(
     return pos_kwargs
 
 
-def is_scalar(value: Any, include_0d: bool = True) -> TypeGuard[Hashable]:
+def is_scalar(value: Any, include_0d: bool = True) -> TypeGuardHashable:
     """Whether to treat a value as a scalar.
 
     Any non-iterable, string, or 0-D array