[python-package] fix mypy error about dask._HostWorkers.__eq__()

[jameslamb]

Contributes to #3756.
Contributes to #3867.

Fixes the following mypy error.

python-package/lightgbm/dask.py:47: error: Argument 1 of "__eq__" is incompatible with supertype "object"; supertype defines the argument type as "object"  [override]
python-package/lightgbm/dask.py:47: note: This violates the Liskov substitution principle
python-package/lightgbm/dask.py:47: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
python-package/lightgbm/dask.py:47: note: It is recommended for "__eq__" to work with arbitrary objects, for example:
python-package/lightgbm/dask.py:47: note:     def __eq__(self, other: object) -> bool:
python-package/lightgbm/dask.py:47: note:         if not isinstance(other, _HostWorkers):
python-package/lightgbm/dask.py:47: note:             return NotImplemented
python-package/lightgbm/dask.py:47: note:         return <logic to compare two _HostWorkers instances>

[jmoralez]

I just learned about typing.NamedTuple. We can have the _HostWorkers class inherit from that and we wouldn't need the __eq__, it works the same way as dataclass with some slight differences (stackoverflow reference).

[jameslamb]

I mentioned that in the description of #3756

I chose to use a regular Python class instead of dataclasses.dataclass to preserve support for Python 3.6 (#5765 (comment)), and chose to avoid typing.NamedTuple (https://stackoverflow.com/a/50038614/3986677) to reduce the risk of incompatibilities caused by future updates to typing.

I'm nervous about introducing a dependency on typing for runtime (not just type-checking) behavior.

[jameslamb]

sorry,I meant #5766

[jmoralez]

Oh, sorry

[jameslamb]

No problem at all! I know having to also have this __eq__ feels like a lot just for slightly better type-checking on two small variables. In a future where we drop Python 3.6 support, this definitely should be converted to a dataclasses.dataclass (but to be clear, I still agree with you that something bigger needs to happen to cause us to drop Python 3.6 support).

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