Account for += assignment when deciding whether to split string

[bbugyi200]

Fixes #2294

[JelleZijlstra]

Should this also cover other augmented assignment operators? I don't think any appear with strings in practice much though, maybe %=.

Thanks for the PR!

[Jackenmen]

I'm not sure how often pathlib.Path is used with the augmented assignment operator rather than the regular one (/) nor do I know how often the string would be long enough that it would need to be split but all that aside, /= can definitely appear with strings on the right-hand of the assignment operator too:

from pathlib import Path
x = Path.cwd()
x /= "directory/subdirectory/file.txt"