Rename some constants and relax pylint rules

For the type system, we want a slightly relaxed rule for constant
naming, so we update the pylint rule. But the old _TPInt and _TNEString
were not clear enough, so we expand them.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>