Fix datamodel_code_generator exception in connector build

[tuliren]

The connector build failed with the following exception:

> Task :airbyte-cdk:python:generateProtocolClassFiles FAILED
Traceback (most recent call last):
  File "/usr/local/bin/datamodel-codegen", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.9/site-packages/datamodel_code_generator/__main__.py", line 329, in main
    pyproject_toml_path = root / "pyproject.toml"
TypeError: unsupported operand type(s) for /: 'tuple' and 'str'

After inspecting the source code of datamodel_code_generator, it appeared that the root variable returned from the black library had a wrong type. Interestingly, black just had a new version, 22.1.0, a few hours ago, which coincided with the time this build started to fail. I compared the version of black pulled by pip when the build was working, and it was 21.12b0. So the new version was probably the root cause of the invalid type. This exception was gone after I pinned black to the old version. It could still fail because of a flaky test in airbyte-worker. But that's a different problem.