string_processing: Better whitespace allocation for an assert statement

[st-pasha]

Describe the style change

Strange indentation of a long assert statement, in --preview (it works correctly in the stable branch).

Examples in the current (preview) Black style

assert (
    result
    == "Lorem ipsum dolor sit amet,\n"
    "consectetur adipiscing elit,\n"
    "sed doeiusmod tempor incididunt\n"
)

Desired style

assert result == (
    "Lorem ipsum dolor sit amet,\n"
    "consectetur adipiscing elit,\n"
    "sed doeiusmod tempor incididunt\n"
)

Additional context

Similar to #3409

[st-pasha]

I think a reasonable rule for the assert statement could be something like this:

• if an assertion has the form

  assert expr1 op expr2

  where op is one of ==, !=, is, is not, in, not in; and the subexpression assert expr1 op fits inside a line length, then prefer to keep that subexpression unbroken, and apply line-breaking to expr2 only.

• if an assertion has the form

  assert expression, explanation

  where the subexpression assert expression fits inside a line length, then prefer to keep that subexpression unbroken, and apply line-breaking to explanation only.