Support _replace (etc.) with namedtuple #661

Reported by Dedoig:

Mypy disallows usage of _replace() method from namedtuple (and likewise for NamedTuple).

from collections import namedtuple
Person = namedtuple("Person", 'name')
p = Person("bob")
p._replace(name="sam")

==> "Person" has no attribute "_replace"

Also, these are missing:

• _asdict()
• _fields