Use importlib-metadata backport for getting version

tests/cli/test_cli.py

@@ -1,10 +1,10 @@
 from click.testing import CliRunner
+from importlib_metadata import version
 
-import textual
 from textual.cli.cli import run
 
 
 def test_cli_version():
     runner = CliRunner()
     result = runner.invoke(run, ["--version"])
-    assert textual.__version__ in result.output, "You need to update __version__"
+    assert version("textual") in result.output