Incorrect stacklevel for deprecate_multichannel_kwarg deprecation warnings.

[Carreau]

Description

deprecate_multichannel_kwarg and alike properly set the stacklevel to 2,
But for functions with this decorator are also decorated with @utils.channel_as_last_axis(), which adds at 1 level between the caller and the final warning location.

This mean that 1) the warning messages are less useful, they all point to skimage/_shared/utils.py:338.
2) filter set for dependees turn warnings into errors don't become error (e.g Napari)

Way to reproduce

# foo.py
from skimage.transform import pyramid_gaussian
pyramid_gaussian(None, multichannel=True)

$ python foo.py
/Users/bussonniermatthias/dev/scikit-image/skimage/_shared/utils.py:338: FutureWarning: `multichannel` is a deprecated argument name for `pyramid_gaussian`. It will be removed in version 1.0.Please use `channel_axis` instead.
  return func(*args, **kwargs)

Version information

e37fc66 (dec 15th), orther info

deprecate_multichannel_kwarg could gain and "extra_stacklevel" to properly place the warning where relevant.

[anntzer]

I haven't checked the skimage internals wrt. warnings, but you may be interested in how we're doing it on matplotlib's side:
https://github.com/matplotlib/matplotlib/blob/a35921c407aced5e65d39b29fd942676ee03679b/lib/matplotlib/_api/__init__.py#L287-L306

def warn_external(message, category=None):
    """
    `warnings.warn` wrapper that sets *stacklevel* to "outside Matplotlib".

    The original emitter of the warning can be obtained by patching this
    function back to `warnings.warn`, i.e. ``_api.warn_external =
    warnings.warn`` (or ``functools.partial(warnings.warn, stacklevel=2)``,
    etc.).
    """
    frame = sys._getframe()
    for stacklevel in itertools.count(1):  # lgtm[py/unused-loop-variable]
        if frame is None:
            # when called in embedded context may hit frame is None
            break
        if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.(?!tests\.))",
                        # Work around sphinx-gallery not setting __name__.
                        frame.f_globals.get("__name__", "")):
            break
        frame = frame.f_back
    warnings.warn(message, category, stacklevel)

[Carreau]

Yeah I had a similar suggestion in #6138.

Those kind of utils should really be in core CPython warnings...