When ConfigParser.items() is called without a section argument, the
implementation delegates to super().items() (i.e. dict.items()) and
returns a collections.abc.ItemsView, not a list. The docs previously
described both overloads as returning a list, which is only correct
for the second overload (with a section argument).

This PR adjusts the first paragraph of the items() documentation in
Doc/library/configparser.rst to say ItemsView instead of list.
The second paragraph (with-section overload) is unchanged because
that one really does return a list.

Verified locally (against Lib/configparser.py on main)

I isolated cpython/Lib/configparser.py from the working tree and
loaded it with importlib.util to confirm the behaviour on current
main (not just the installed interpreter):

import importlib.util
spec = importlib.util.spec_from_file_location(
    "cpy_configparser",
    "/path/to/cpython/Lib/configparser.py",
)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)

cp = mod.ConfigParser()
cp.read_string("[sec]\nkey=val")
print(type(cp.items()).__name__)        # ItemsView
print(type(cp.items("sec")).__name__)   # list

Result:

no_arg type:  collections.abc.ItemsView
with_arg type: builtins.list
no_arg is collections.abc.ItemsView? True
with_arg is list? True

git log -L:'def items':Lib/configparser.py shows the body of
items() has not been touched since 199507b81a (a docstring-only
rst-markup fix), so this is a long-standing docs/behaviour mismatch
rather than a recent regression.

No code change. No Misc/NEWS.d entry (docs-only).

Fixes #149050

• Issue: Note that configparser.ConfigParser.items returns a collections.abc.ItemsView in the first overload #149050