[python-package] use dataclass for CallbackEnv

[jameslamb]

Contributes #3756.
Contributes to #3867.

Proposes converting CallbackEnv in the Python package to a dataclass.

Also proposes removing uses of import collections in favor of importing only exactly what LightGBM needs from that module.

Restating the benefits I mentioned when I last tried this in #5765, dataclasses are preferable to namedtuple in the following ways:

• easier to set type annotations + default values than named tuples
  • makes it easier for both human developers and and tools like mypy to understand the expected types of properties
• possible to attach other methods or computed properties with @property (just like normal classes)

I realized today that "use dataclassses" and "drop Python 3.6 support" don't have to be coupled... I was wrong to suggest that coupling in #5765 and #6022 (comment).

This proposes taking on the dataclasses package as a required dependency only for Python 3.6. For details on that mechanism, see PEP 508 and PEP 631:

• https://peps.python.org/pep-0508/
• https://peps.python.org/pep-0631/

That package still exists on PyPI exactly for cases like this... it's intended to be used to continue to provide dataclasses support for Python 3.6: https://pypi.org/project/dataclasses/.

[jameslamb]

Thanks @jmoralez ! Sorry I didn't think of this back in #5765.

[jmoralez]

No worries. I forgot about that pip package as well haha

[github-actions]

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