Cannot import altair Chart in python 3.14

### What happened?

I'm testing python 3.14 with altair and get the following error:

```sh
    from altair import Chart
  File ".venv/lib/python3.14/site-packages/altair/__init__.py", line 649, in <module>
    from altair.vegalite import *
  File ".venv/lib/python3.14/site-packages/altair/vegalite/__init__.py", line 2, in <module>
    from .v5 import *
  File ".venv/lib/python3.14/site-packages/altair/vegalite/v5/__init__.py", line 2, in <module>
    from altair.expr.core import datum
  File ".venv/lib/python3.14/site-packages/altair/expr/__init__.py", line 11, in <module>
    from altair.expr.core import ConstExpression, FunctionExpression
  File ".venv/lib/python3.14/site-packages/altair/expr/core.py", line 6, in <module>
    from altair.utils import SchemaBase
  File ".venv/lib/python3.14/site-packages/altair/utils/__init__.py", line 1, in <module>
    from .core import (
    ...<9 lines>...
    )
  File ".venv/lib/python3.14/site-packages/altair/utils/core.py", line 22, in <module>
    from altair.utils.schemapi import SchemaBase, SchemaLike, Undefined
  File ".venv/lib/python3.14/site-packages/altair/utils/schemapi.py", line 29, in <module>
    from typing_extensions import TypeAlias
ModuleNotFoundError: No module named 'typing_extensions'
```

### What would you like to happen instead?

no error

### Which version of Altair are you using?

altair 5.5.0 with python 3.14-rc3

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