Multi-lines with block should be allowed to use backslashes to wrap their lines

[seirl]

Describe the style change Multi-lines with block should be allowed to use backslashes to wrap their lines.

Examples in the current Black style

        with ZSTFile(nodes_file, "w") as nodes_writer, ZSTFile(
            edges_file, "w"
        ) as edges_writer:

Desired style

        with \
                ZSTFile(nodes_file, 'w') as nodes_writer, \
                ZSTFile(edges_file, 'w') as edges_writer:

Additional context Add any other context about the problem here.

This is a well-known place where backslashes are a valid form of line continuation in Python. It makes the separation between lines a lot clearer.

I know this could be rewritten using ExitStack, but I don't think this is a good reason for Black to generate unreadable lines by default when people are not using ExitStack.

[intgr]

Unfortunately there isn't a nicer way to format with blocks.

I really wish Python allowed optional parentheses in the with syntax, like this

        with (
            ZSTFile(nodes_file, 'w') as nodes_writer,
            ZSTFile(edges_file, 'w') as edges_writer,
        ):

[seirl]

Duplicate of #664 apparently.