Don't split type annotation with alternative union syntax

[cdce8p]

Describe the style change
With postponed evaluations of type annotations it's now possible to write a Union as int | None. With black 20.8b1 this can sometimes be formatted in a misleading way.

Examples in the current Black style

from __future__ import annotations

def f():
    args: None
    | tuple = None,  # this is a really long comment with some extra information

Desired style

from __future__ import annotations

def f():
    args: (
        None | tuple
    ) = None,  # this is a really long comment with some extra information

Additional context
With from __future__ import annotations, this syntax is valid in Python 3.7+
https://www.python.org/dev/peps/pep-0604/

[ambv]

Note: the example isn't about variable annotations where this sort of split would be illegal.

It's about function arguments:

def f(
    args: tuple 
    | None = None,
):
    ...

Indeed, it would be nicer to format like what Black does for variable annotations:

-    some_argument: tuple | None = None  # very very very very very very very very very very very very
+    some_argument: tuple | None = (
+        None  # very very very very very very very very very very very very
+    )

Note that if there is no assignment, Black doesn't even attempt a split on a line that's too long:

some_argument: tuple | None  # very very very very very very very very very very very very

It's left as is.

[JelleZijlstra]

Duplicate of #2316