The attribute name, '_fields_defaults' was misspelled and should have been ''_field_defaults'.  The namedtuple documentation uses both spellings.  The typing.NamedTuple class consistently uses the latter spelling.  The intent was the both would be spelled the same way.

    >>> from typing import NamedTuple
    >>> class Employee(NamedTuple):
        name: str
        id: int = 3
    >>> Employee._field_defaults
    {'id': 3}

    >>> from collections import namedtuple
    >>> Employee = namedtuple('Employee', ['name', 'id'], defaults=[3])
    >>> Employee._fields_defaults
    {'id': 3}

Since 3.7 API is already released, it may be reasonable to provide both spellings for namedtuple().

New changeset 23581c018fceb607fe829a41c6fbe81b4d502cab by Raymond Hettinger in branch 'master':
bpo-36321: Fix misspelled attribute in namedtuple() (GH-12375)
https://github.com/python/cpython/commit/23581c018fceb607fe829a41c6fbe81b4d502cab

New changeset bedfbc790e18f14cfdd59cf27d27bb86518dc3fc by Raymond Hettinger (Miss Islington (bot)) in branch '3.7':
bpo-36321: Fix misspelled attribute in namedtuple() (GH-12375) (GH-12395)
https://github.com/python/cpython/commit/bedfbc790e18f14cfdd59cf27d27bb86518dc3fc

New changeset 58ccd201fa74287ca9293c03136fcf1e19800ef9 by Raymond Hettinger in branch 'master':
bpo-36321: Fix misspelled attribute name in namedtuple() (GH-16858)
https://github.com/python/cpython/commit/58ccd201fa74287ca9293c03136fcf1e19800ef9