Copy with slicing of a list[subclass] should be ok for list[superclass]

• Are you reporting a bug, or opening a feature request?
  As I expected this to work, I assume it is a bug.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import List

class Upi(object):
    pass


class Subi(Upi):
    pass


subs = [Subi(), Subi()]
ok1:List[Upi] = [s for s in subs]
ok2:List[Upi] = list(subs)
notOk:List[Upi] = subs[:] # this should also be ok IMHO

• What is the actual behavior/output?
  notOk list is not ok for mypy.

• What is the behavior/output you expect?
  Should be ok. Actually saw this in a function return value so should be ok there also when function is returning list of superclass things.

• What are the versions of mypy and Python you are using?
  mypy 0.720, Python 3.7.3

• What are the mypy flags you are using? (For example --strict-optional)
  None

The issue names no implementation files or tests. Begin by running the supplied repro with mypy, then trace how list slicing is typed compared with a comprehension and list(subs). Done means assigning subs[:] to List[Upi] is accepted without flags, with coverage for the reported case.