Crashes with isort<5

[akaihola]

$ pip install 'isort<5' darker==1.2.4
$ darker --isort .
Traceback (most recent call last):
  File "bin/darker", line 5, in <module>
    from darker.__main__ import main
  File "darker/__main__.py", line 23, in <module>
    from darker.import_sorting import apply_isort, isort
  File "darker/import_sorting.py", line 21, in <module>
    isort_code = getattr(isort, "code")
AttributeError: module 'isort' has no attribute 'code'