Formatter: wrap_multiple_context_managers_in_parens preview style

[MichaReiser]

Implement Black's wrap_multiple_context_managers_in_parens as a preview style. The new style only applies to Py39+.

with \
     make_context_manager1() as cm1, \
     make_context_manager2() as cm2, \
     make_context_manager3() as cm3, \
     make_context_manager4() as cm4 \
:
    pass

Gets formatted as

with (
    make_context_manager1() as cm1,
    make_context_manager2() as cm2,
    make_context_manager3() as cm3,
    make_context_manager4() as cm4,
):
    pass

Black automatically detects the python version used in a file based on the syntax used. E.g. a file using match statements is Py310+. I'm unsure if we should autodetect the python version based on the syntax or rely on requires-python

[charliermarsh]

I'm unsure if we should autodetect the python version based on the syntax or rely on requires-python.

I would prefer to stick to our existing configuration semantics, and not introduce any autodetection.

[charliermarsh]

It looks like the original intent, per the docs, is to reformat as:

# Input
with make_context_manager1() as cm1, make_context_manager2() as cm2, make_context_manager3() as cm3, make_context_manager4() as cm4:
    pass

# < Python 3.9
with \
     make_context_manager1() as cm1, \
     make_context_manager2() as cm2, \
     make_context_manager3() as cm3, \
     make_context_manager4() as cm4 \
:
    pass

# >= Python 3.9
with (
    make_context_manager1() as cm1,
    make_context_manager2() as cm2,
    make_context_manager3() as cm3,
    make_context_manager4() as cm4,
):
    pass

But AFAICT Black only implemented the second piece, and not the continuation escapes.