Rewriting six.raise_from breaks with multiline args

[hugovk]

pyupgrade==2.7.3

Running pyupgrade 1.py --py3-plus

With Black magic comma

Before

import six

six.raise_from(
    ValueError("a long error message which makes Black add a magic comma after None"),
    None,
)

After

syntax error:

import six

raise ValueError("a long error message which makes Black add a magic comma after None") from 
    None,

Without Black magic comma

Before

import six

six.raise_from(
    ValueError("a long error message which makes Black add a magic comma after None"),
    None
)

After

syntax error:

import six

raise ValueError("a long error message which makes Black add a magic comma after None") from 
    None

Both args on same line

Before

import six

six.raise_from(
    ValueError("a long error message which makes Black add a magic comma after None"), None
)

After

Now it's okay:

import six

raise ValueError("a long error message which makes Black add a magic comma after None") from None

[asottile]

probably the "best" way to fix this is with backslash insertion, as ugly as it is

[hugovk]

In this particular case, Black's multilines triggered the bug, and to atone will happily mop up any ugly backslashes afterwards :)

[asottile]

via #361