test_no_typing_extensions_common_combined fails due to thrid-party modules

[mattpap]

Fails with:

__________________ test_no_typing_extensions_common_combined ___________________

    def test_no_typing_extensions_common_combined() -> None:
        ''' Basic usage of Bokeh should not result in typing_extensions being
        imported. This test ensures that importing basic modules does not bring in
        typing_extensions.
    
        '''
        proc = run([python, "-c", verify_clean_imports('typing_extensions', MODULES)])
>       assert proc.returncode == 0, "typing_extensions imported in common modules"
E       AssertionError: typing_extensions imported in common modules
E       assert 1 == 0
E        +  where 1 = CompletedProcess(args=['/usr/share/miniconda3/envs/bk-test/bin/python3.9', '-c', "\nimport sys\nfor module in ['bokeh'...xtensions') for key in sys.modules.keys()):\n        print(module)\n        sys.exit(1)\nsys.exit(0)\n"], returncode=1).returncode

tests/codebase/test_no_typing_extensions_common.py:[50]

I tracked this down to dependencies of PIL and sphinx.