Modernize annotation usage in libregrtest

Bug description:

Lib/test/libregrtest/results.py imports TYPE_CHECKING only to guard a type-only import:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from xml.etree.ElementTree import Element

Now that explicit lazy imports are available, this can be replaced with:

lazy from xml.etree.ElementTree import Element

The stringized list['Element'] annotation can then become list[Element]. This keeps xml.etree.ElementTree from being imported eagerly while making the annotation name available at runtime.

Lib/test/libregrtest/mypy.ini should also target Python 3.15, following the approach used for pyrepl in gh-149985 and gh-149992.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-156403

Start with Lib/test/libregrtest/results.py and Lib/test/libregrtest/mypy.ini; inspect the existing annotation and type-checking configuration, then compare the pyrepl approach in gh-149985 and gh-149992. Done means the lazy annotation import and runtime annotation are used without eager XML imports, and mypy targets Python 3.15.