Prohibit some illegal uses of Literal

[Michael0x2a]

This pull request checks to make sure we report errors if the user tries using Literal types in invalid places. In particular, this PR...

1. Adds tests to make sure we cannot subclass Literals (mypy already directly supported this)
2. Checks to make sure we cannot use Literals inside isinstance and issubclass checks

I also wanted to add a check preventing people from attempting to instantiate a Literal (e.g. disallow Literal[3]() or Literal()), but that might require a little more work and a few changes to typing_extensions. (We currently don't raise an error when people try doing things like Final(), Final[int]() or Protocol() either).

[ilevkivskyi]

(We currently don't raise an error when people try doing things like Final(), Final[int]() or Protocol() either)

Hm, maybe this depend on version, but I see this:

>>> Protocol()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ivan/mypy_dev/lib/python3.6/site-packages/typing_extensions.py", line 1166, in __new__
    raise TypeError("Type Protocol cannot be instantiated; "
TypeError: Type Protocol cannot be instantiated; it can be used only as a base class
>>> Final()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/typing.py", line 194, in __call__
    raise TypeError("Cannot instantiate %r" % type(self))
TypeError: Cannot instantiate typing_extensions.Final
>>> Final[int]()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/typing.py", line 194, in __call__
    raise TypeError("Cannot instantiate %r" % type(self))
TypeError: Cannot instantiate typing_extensions.Final

Anyway, I think the fact that the error is emitted at runtime means it is not super-important to also flag this statically.

[Michael0x2a]

(We currently don't raise an error when people try doing things like Final(), Finalint or Protocol() either)

Oh, I meant we don't raise an error at type-check-time. (Well, we sort of do for Final(), Protocol(), and Literal() -- but the error message is just "_SpecialForm not callable", which I imagine would somewhat puzzle users who aren't aware of mypy/typing internals.)

Anyway, I think the fact that the error is emitted at runtime means it is not super-important to also flag this statically.

Yeah -- I agree. I thought about making more of an effort to add support for this, but came to the same conclusion you did and just submitted this PR as-is.

[ilevkivskyi]

Thanks! LGTM.