Fix TypeVar for decorators that handle Commands

[euresti]

The old type var would throw away the types of Callables.

import click

@click.option("--name", prompt="Your name", help="The person to greet.")
def hello(name: str) -> None:
    ...

reveal_type(hello)   #  note: Revealed type is 'def (*Any, **Any) -> Any'

With this small adjustment mypy now knows that it's a 'def (name: builtins.str)'

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.