Minimum typing-extensions version is not specified

### Checklist

- [X] The bug is reproducible against the latest release or `master`.
- [X] There are no similar issues or pull requests to fix it yet.

### Describe the bug

Starlette has a bare `typing-extensions` requirement, with no minimum version. But using older versions of `typing-extensions` will cause an error.

### Steps to reproduce the bug

```

pip install typing-extensions~=3.7.0

pip install starlette

python -c "from starlette.applications import Starlette"

```

### Expected behavior

Starlette should specify the minimum version of `typing-extensions` it requires, which would cause the `pip install starlette` command to upgrade `typing-extensions` to the required version (3.10).

### Actual behavior

Starlette doesn't install the required `typing-extensions` version, then the import fails.

### Debugging material

```

$ python -c "from starlette.applications import Starlette"

Traceback (most recent call last):

  File "<string>", line 1, in <module>

  File ".../lib/python3.8/site-packages/starlette/applications.py", line 3, in <module>

    from starlette.datastructures import State, URLPath

  File ".../lib/python3.8/site-packages/starlette/datastructures.py", line 8, in <module>

    from starlette.concurrency import run_in_threadpool

  File ".../lib/python3.8/site-packages/starlette/concurrency.py", line 10, in <module>

    from typing_extensions import ParamSpec

ImportError: cannot import name 'ParamSpec' from 'typing_extensions' (.../lib/python3.8/site-packages/typing_extensions.py)

```

### Environment

- Ubuntu 20.04, Python 3.8, Starlette 0.18

### Additional context

_No response_

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