🐛 Fix usage of Annotated with future annotations in Python 3.8

Fixes #802

`RuntimeError: Type not yet supported: typing_extensions.Annotated[...]`

The solution is to use the `get_type_hints` method from `typing_extensions`
instead of the built-in in the `typing` module, as it includes backports that
support properly evaluating the forward reference.

tests/test_future_annotations.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+import typer
+from typer.testing import CliRunner
+from typing_extensions import Annotated
+
+runner = CliRunner()
+
+
+def test_annotated():
+    app = typer.Typer()
+
+    @app.command()
+    def cmd(force: Annotated[bool, typer.Option("--force")] = False):
+        if force:
+            print("Forcing operation")
+        else:
+            print("Not forcing")
+
+    result = runner.invoke(app)
+    assert result.exit_code == 0, result.output
+    assert "Not forcing" in result.output
+
+    result = runner.invoke(app, ["--force"])
+    assert result.exit_code == 0, result.output
+    assert "Forcing operation" in result.output

typer/utils.py

@@ -1,9 +1,9 @@
 import inspect
 import sys
 from copy import copy
-from typing import Any, Callable, Dict, List, Tuple, Type, cast, get_type_hints
+from typing import Any, Callable, Dict, List, Tuple, Type, cast
 
-from typing_extensions import Annotated
+from typing_extensions import Annotated, get_type_hints
 
 from ._typing import get_args, get_origin
 from .models import ArgumentInfo, OptionInfo, ParameterInfo, ParamMeta