Fold nested context managers.

[shaleh]

Context managers can be grouped in the same way as imports.

with foo:
    with bar:
        body

becomes

with (foo, bar):
    body

Resolves. https://github.com/asottile/pyupgrade/issues/872

[shaleh]

PR updated as requested. In doing so I found a case I had fixed and then somehow missed during subsequent debugging.

[shaleh]

I would appreciate insight into why the 3.9 code coverage is failing. It passes for 3.10.