Issue 2440: remove the guard around the handling of case 'n' in getargs.c's convertsimple() such that we always treat it as an index type, regardless of whether or not sizeof(size_t) == sizeof(long).  Fix the test_args2.Signed_TestCase.test_n() such that it tests for adherence to PEP 357 (don't try and coerce objects that don't have nb_index slots but do have nb_int slots (i.e. floats) into indexes 'just because we can').  Three other commits are related to this one: r62269 and r62279, which were changes to PyNumber_Index (among other things) to check for nb_int slots when we lack nb_index slots -- and r62292, which is when I reverted these changes after various people pointed out that the test was in fact wrong, not the code.