[Python-ideas] Make "is" checks on non-singleton literals errors

I regularly see learners using "is" to check for string equality and sometimes other equality. Due to optimizations, they often come away thinking it worked for them. There are no cases where if x is "foo": or if x is 4: is actually the code someone intended to write. Although this has no benefit to anyone but new learners, it also doesn't really do any harm. Mike