This diff adds support for performing reachability and narrowing
analysis when doing certain enum checks.

For example, given the following enum:

    class Foo(Enum):
        A = 1
        B = 2

...this pull request will make mypy do the following:

    x: Foo
    if x is Foo.A:
        reveal_type(x)  # type: Literal[Foo.A]
    elif x is Foo.B:
        reveal_type(x)  # type: Literal[Foo.B]
    else:
        reveal_type(x)  # No output: branch inferred as unreachable

This diff does not attempt to perform this same sort of narrowing for
equality checks: I suspect implementing those will be harder due to
their overridable nature. (E.g. you can define custom `__eq__` methods
within Enum subclasses).

This pull request also finally adds support for the enum behavior
[described in PEP 484][0] and also sort of partially addresses
python#6366

  [0]: https://www.python.org/dev/peps/pep-0484/#support-for-singleton-types-in-unions

Co-Authored-By: Ran Benita <ran234@gmail.com>

…mypy into refine-enum-branch-analysis