Preview style: async with context managers aren't wrapped in parentheses

[yilei]

#3489 wraps multiple context managers in parentheses when targeting Python 3.9+, but async with statements aren't handled.

We noticed this in the review of #3589, but the fix is different enough that I'm filing a separate issue and will send a follow-up PR.

Example: https://black.vercel.app/?version=stable&state=_Td6WFoAAATm1rRGAgAhARYAAAB0L-Wj4AIWAK9dAD2IimZxl1N_WmufDs0LojAMwSd5y6tVHFECi48em1NmrZSWd-aOQDsyxUoj0wG4N0KjynelfrUIzSWL64h5VoX_mCOwgMPULx38--l2ByPDULdy3kY55NpHvpodxXDskAGNjzDUowTiDYEk1Nqn0zKozBat4Jr52JOK7_c0zbTbJlm2bGELr0genqmpOYe0Veq-1N1caP91OmdWsqLjL9f2ZG7iyYFCy51v8cbzDGQAACCNqwSsCCbzAAHLAZcEAADbYoSiscRn-wIAAAAABFla