gh-80527: Change support.requires_legacy_unicode_capi() (#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.