BUCK python3 parser unable to build dist-upgrader/pleskdistup:config

[SandakovMM]

Describe the bug

Unable to build converter/distupgrader by buck when parser in .buckconfig is python3 or not set.
In this case build fails with:

Buck wasn't able to parse /home/builder/msandakov/ubuntu18to20/ubuntu18to20/BUCK:
TypeError: keys must be a string
Call stack:
  File "/usr/lib64/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib64/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)

STR

1. Take ubuntu18to20
2. Change .buckconfig python_interpreter to /usr/bin/python3
3. buck build :ubuntu18to20

Additional information

It looks like the problem is in the format call. Removing it helps to avoid the problem.