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

[Mike Graham]

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