The **attrs** and **cattrs** packages version is incompatible.
Check command:
`pip check --use-feature=2020-resolver`

Code file: https://github.com/apache/airflow/blob/master/setup.py
Commit hash: 9150330
Line number:
694: 'attrs~=19.3'
696: 'cattrs~=1.0'

It will caused the final installed **attrs** version is higher than 20.1.0.

Why this happen?
From the dependency setting of **cattrs**:
https://github.com/Tinche/cattrs/blob/master/setup.py
Commit hash: 2f401b9
Line number:
12: "attrs >= 20.1.0"

setup.py
"""Test suite for Airflow tests"""
    test_loader = unittest.TestLoader()
    test_suite = test_loader.discover(os.path.join(my_dir, 'tests'), pattern='test_*.py')
    return test_suite
class CleanCommand(Command):
    """
    Command to tidy up the project root.
    Registered as cmdclass in setup() so it can be called with ``python setup.py extra_clean``.
    """
    description = "Tidy up the project root"
    user_options = []  
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):
        """Set final values for options."""
    def run(self):  
        """Run command to remove temporary files and directories."""
        os.chdir(my_dir)
        os.system('rm -vrf ./build ./dist ./*.pyc ./*.tgz ./*.egg-info')
class CompileAssets(Command):