Add typing_extensions to dependencies.

Author: dcherian

pyproject.toml

@@ -19,7 +19,12 @@ name = "xarray"
 readme = "README.md"
 requires-python = ">=3.10"
 
-dependencies = ["numpy>=1.24", "packaging>=23.2", "pandas>=2.1"]
+dependencies = [
+  "numpy>=1.24",
+  "packaging>=23.2",
+  "pandas>=2.1",
+  "typing_extensions>=4.8",
+]
 
 # We don't encode minimum requirements here (though if we can write a script to
 # generate the text from `min_deps_check.py`, that's welcome...). We do add