PEP8 backslash exception for `with` statements is not honored

[ergoithz]

Given this example, explicitly allowed by PEP8 (lit. Backslashes may still be appropriate at times. For example, long, multiple with-statements cannot use implicit continuation, so backslashes are acceptable):

async def fnc():
    async with asdf() as a, \
               bsdf():
        return 1

with asdf() as a, \
     bsdf():
    return 1


with \
  asdf() as a, \
  bsdf():
    return 1

This plugin gives many non-PEP8-compliant false positives:

test_continuation.py:2:1: N400: Found backslash that is used for line breaking
    async with asdf() as a, \
               bsdf():
^
test_continuation.py:6:1: N400: Found backslash that is used for line breaking
with asdf() as a, \
     bsdf():
^
test_continuation.py:11:1: N400: Found backslash that is used for line breaking
with \
  asdf() as a, \
  bsdf():
^
test_continuation.py:12:3: N400: Found backslash that is used for line breaking
with \
  asdf() as a, \
  bsdf():
  ^

[153957]

At least we can now (Python 3.10+) use parentheses in context expressions:

• https://docs.python.org/3/whatsnew/3.10.html#parenthesized-context-managers
• Multiple context expressions do not support parentheses for continuation across lines python/cpython#56991