🐛 Fix evaluating stringified annotations in Python 3.8 & 3.9 (also from __future__ import annotations)

[sisp]

I've fixed evaluating stringified annotations in Python 3.9 and less when using the Annotated[<type>, typer.{Argument,Option}(...)] syntax. When removing the @needs_py310 decorator from the test_annotated_argument_in_string_type_with_default test, it fails with the following error:

――――――――――――― test_annotated_argument_in_string_type_with_default ――――――――――――――
[gw0] linux -- Python 3.9.20 .../typer/.venv/bin/python3

    def test_annotated_argument_in_string_type_with_default():
        app = typer.Typer()
    
        @app.command()
        def cmd(val: "Annotated[int, typer.Argument()]" = 0):
            print(f"hello {val}")
    
        result = runner.invoke(app)
        assert result.exit_code == 0, result.output
        assert "hello 0" in result.output
    
        result = runner.invoke(app, ["42"])
>       assert result.exit_code == 0, result.output
E       AssertionError: Usage: cmd [OPTIONS]
E         Try 'cmd --help' for help.
E         
E         Error: Got unexpected extra argument (42)
E         
E       assert 2 == 0
E        +  where 2 = <Result SystemExit(2)>.exit_code

tests/test_annotated.py:38: AssertionError

This problem typically occurs when from __future__ import annotations is used with Python prior to 3.10.

This PR fixes the problem for Python 3.8 & 3.9 by evaluating string annotations with typing_extensions.get_annotations(...) (added in v4.13.0) instead of inspect.signature(..., eval_str=True), which is only available for Python 3.10+. I can't get this to work for Python 3.7, which is still declared as supported despite having reached EOL more than 2 years ago, as typing-extensions v4.13.0 requires Python 3.8+. Nevertheless, this PR fixes the above problem for all non-EOL Python versions and even Python 3.8.

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