BUG: extra " " inserted when reformatting long string literals

[jbrockmendel]

See e.g. https://github.com/pandas-dev/pandas/pull/28857/files

Before using black, we have something like:

raise NotImplementedError(
        "foo bar"
        "baz"
    )

After using black, the string is reformatted with an unwanted " ":

raise NotImplementedError("foo bar" "baz")

Whereas we would want

raise NotImplementedError("foo bar baz")

Would a PR addressing this be accepted?

[JelleZijlstra]

This isn't really a bug, since the output still behaves the same, it's just suboptimal formatting. There is already an enhancement request for merging implicitly concatenated strings on the same line: #26.