Fix mypy errors from deprecated click.MultiCommand isinstance check

[adamtheturtle]

Fixes #202.

click.MultiCommand is deprecated since Click 8 and is no longer usable as the second argument to isinstance at the type level, causing mypy cloup to fail on master with two errors in cloup/_commands.py. click.Group exposes format_commands on every supported Click version (>= 8.0), so the behavior at runtime is unchanged. This also silences the DeprecationWarning emitted when format_help is called on a group.

In practice, the only MultiCommand cloup ever produces is cloup.Group (which subclasses click.Group), so the original check was already effectively a Group check.

Test plan

Tested across the full supported Click range (>= 8.0, < 9.0), Python 3.12, mypy 2.1.0:

Click	pytest	mypy --strict cloup
8.0.4	3 failures (pre-existing on master)	2 errors (pre-existing on master)
8.1.8	328 pass	clean
8.2.1	328 pass	clean
8.3.3	328 pass	clean
8.4.1	328 pass	1 error in _params.pyi (fixed by #201)

The Click 8.0 issues reproduce on master without this change and are unrelated:

• pytest: cloup/types.py passes executable=... to click.Path, a kwarg added in a later Click 8.x
• mypy: two # type: ignore comments on @overload decorators (lines 273, 350) become unused because the underlying mypy warning doesn't fire against Click 8.0's older stubs

🤖 Generated with Claude Code

[janluke]

This change would break compatibility with click 8.1. Think of a user on click 8.1 who still uses a MultiCommand that is not a Group. For such user, Cloup would silently skip the commands section because a MultiCommand is not a Group. I'll fix this separately.

Easiest way is to require click >= 8.2. In that case, everything's fine because _MultiCommand subclasses Group. Nonetheless, I still need to support 8.1 because that's the only version that works in Python 3.9 and I can't drop support at this time (even if it's planned) as it breaks a bunch of stuff (e.g. docs with very old dependencies that I have no time to fix now).

So I will do this instead:

• I requires click >= 8.1. This will make it impossible the bug with 8.0.
• I'll replace the isinstance check in this PR with just __has_attr__(self, "format_commands") for now.

Closing this PR, as I'll apply the above changes instead. But thank you very much for this!

[adamtheturtle]

Thank you!