Implement additional checks for namedtuple

[erictraut]

This PR proposes to amend the typing spec and clarify that type checkers should disallow named tuple field names that begin with an underscore. I wasn't previously aware of this limitation, so pyright doesn't enforce this. Since this will likely become mandated by the typing spec, it should be added.

[erictraut]

This is addressed in 1.1.400.