Enable intersphinx so stdlib references resolve

[faresrafat3]

Fixes #192 (partially).

Building the docs with sphinx-build -n reports many warnings of the form:

<unknown>:1: WARNING: py:class reference target not found: collections.abc.Mapping [ref.class]
<unknown>:1: WARNING: py:class reference target not found: datetime.datetime [ref.class]
<unknown>:1: WARNING: py:class reference target not found: collections.abc.Sequence [ref.class]

The cause is that docs/conf.py only enables sphinx.ext.autodoc; with no
intersphinx mapping, references to the standard library cannot be resolved.

Change

Adds sphinx.ext.intersphinx and the Python inventory:

extensions = [
    "sphinx.ext.autodoc",
    "sphinx.ext.intersphinx",
]

intersphinx_mapping = {
    "python": ("https://docs.python.org/3", None),
}

Verification

Measured with the same command on the unmodified and modified tree:

before: 55 warnings
after:  31 warnings

All collections.abc.* and datetime.* warnings are gone. The remaining 31
are project-internal targets (tomlkit.items.ItemT,
tomlkit.container.Container, E, Encoder, and a docutils formatting
warning in the register_encoder docstring) which need separate fixes and are
not addressed here.

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