_legacy: mark legacy API as PendingDeprecationWarning

[FFY00]

Per #80

[jaraco]

On further consideration, I believe the DeprecationWarning is working as intended. In particular, the docs say:

Base category for warnings about deprecated features when those warnings are intended for other Python developers (ignored by default, unless triggered by code in __main__).

In other words, the warning should not appear for users and library maintainers should have the ability to update to account for the change over months (the intention of the deprecation). Here's a demo:

draft $ cat mod.py
__requires__ = ['importlib_resources >=5.3']

import contextlib
import importlib_resources as res

with contextlib.suppress(Exception):
    res.is_resource('mod', 'bar')
draft $ pip-run -q importlib_resources -- -c "import mod; print('done')"
done

But the warning is raised if found in __main__:

draft $ pip-run -q importlib_resources -- -m mod
/Users/jaraco/draft/mod.py:7: DeprecationWarning: is_resource is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
  res.is_resource('mod', 'bar')

The legacy interface is in fact deprecated and superseded by a superior interface and there's a nice migration path.

Let's stick with the DeprecationWarning for now unless we get more clarity from the SC that the functionality should not be deprecated in 3.11.

[FFY00]

Alright, 👍