[3.12] gh-80527: Change support.requires_legacy_unicode_capi() (GH-108438) (#108446)

gh-80527: Change support.requires_legacy_unicode_capi() (GH-108438)

The decorator now requires to be called with parenthesis:

    @support.requires_legacy_unicode_capi()

instead of:

    @support.requires_legacy_unicode_capi

The implementation now only imports _testcapi when the decorator is
called, so "import test.support" no longer imports the _testcapi
extension.
(cherry picked from commit 995f4c48e11349fbfb9233e02b732d4534d3008e)

Co-authored-by: Victor Stinner <vstinner@python.org>