Indentation of split string in dict would be better align to the value rather than key

[masterspelling]

Describe the bug
Indentation of split string in dict would be better align to the value rather than key

To Reproduce

For example, take this code:

a_dict = {
    "key1": "value1",
    "key2": "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
       "long long value",
}

Expected behavior

I think it would be visually more pleasing to have it aligned by value

a_dict = {
    "key1": "value1",
    "key2": "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
            "long long value",
}

currently it is aligns it by key

a_dict = {
    "key1": "value1",
    "key2": "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
    "long long value",
}

[jakkdl]

getting it aligned by the value sounds messy, given the length of the key may differ greatly - and esp if the length of the key is very long it makes it incredibly messy

a dict = {
    my_function_call(arg1, arg2, arg3, 300 + 25 * 99): "long longlonglonglonglonglonglonglong"
                                                       "long long value",
    my_function_call(arg1, arg2, arg3): "long longlonglonglonglonglonglonglong"
                                        "long long value",
}

It should probably get wrapped in parentheses (easy to implement, something similar to #3899) or get indented one step further in though:

a_dict = {
    "key1": "value1",
    "key2": "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
        "long long value",
    "key3": (
        "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
        "long long value"
    ),
}

[JelleZijlstra]

We should use the parenthesized version. This follows the principle in Black that indentation should always be a multiple of four.

[Shreya-7]

Happy to take a stab at this!

[JelleZijlstra]

Current preview style adds parentheses:

% black --preview -c '''a_dict = {
    "key1": "value1",
    "key2": "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
       "long long value",
}
'''
a_dict = {
    "key1": "value1",
    "key2": (
        "long longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong"
        " long value"
    ),
}