Remove mentions of old Python versions in typing.NamedTuple docstring

Documentation

This came up in this PR
In typing.NamedTuple there are some mentions to Python 3.5 & 3.6 which can be removed:

Line 2712 in bd1b622

Usage in Python versions >= 3.6::

Lines 2729 to 2731 in bd1b622

    
               In Python versions <= 3.5 use:: 
        
                   Employee = NamedTuple('Employee', [('name', str), ('id', int)])

	In Python versions <= 3.5 use::

	Employee = NamedTuple('Employee', [('name', str), ('id', int)])