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

從 Lib/test/libregrtest/results.py 和 Lib/test/libregrtest/mypy.ini 開始；檢查現有的註解與型別檢查設定，然後比較 gh-149985 和 gh-149992 中的 pyrepl 作法。完成的標準是使用 lazy annotation import 和 runtime annotation，且不進行 eager XML imports，並讓 mypy 以 Python 3.15 為目標。