Replace pkg_resources with importlib.metadata in docs config

[Copilot]

ReadTheDocs builds fail because pkg_resources is unavailable in newer Python environments where setuptools no longer bundles it by default.

• docs/conf.py: Switch from pkg_resources.get_distribution() to importlib.metadata.version() (stdlib since Python 3.8)
• docs/requirements.txt: Drop setuptools dependency, no longer needed for docs build

# Before
from pkg_resources import DistributionNotFound, get_distribution
__version__ = get_distribution("emcee").version

# After
from importlib.metadata import PackageNotFoundError, version
__version__ = version("emcee")

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