It already mostly worked, except in the case when invalid keyword
argument with non-ASCII name was passed to function with non-ASCII
parameter names. Then it crashed in the debug mode.
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.
