test__osx_support and test_selectors are broken, examples:

https://github.com/python/cpython/pull/21806/checks?check_run_id=966438645

2020-08-10T12:04:20.7613070Z ======================================================================
2020-08-10T12:04:20.7613710Z ERROR: test__remove_unsupported_archs (test.test__osx_support.Test_OSXSupport)
2020-08-10T12:04:20.7616560Z ----------------------------------------------------------------------
2020-08-10T12:04:20.7617240Z Traceback (most recent call last):
2020-08-10T12:04:20.7617860Z   File "/Users/runner/work/cpython/cpython/Lib/test/test__osx_support.py", line 23, in setUp
2020-08-10T12:04:20.7618560Z     self.env = test.support.EnvironmentVarGuard()
2020-08-10T12:04:20.7621650Z AttributeError: module 'test.support' has no attribute 'EnvironmentVarGuard'

and

2020-08-10T12:01:40.0736200Z ======================================================================
2020-08-10T12:01:40.0738440Z ERROR: test_register_bad_fd (test.test_selectors.KqueueSelectorTestCase)
2020-08-10T12:01:40.0742700Z ----------------------------------------------------------------------
2020-08-10T12:01:40.0744640Z Traceback (most recent call last):
2020-08-10T12:01:40.0747460Z   File "/Users/runner/work/cpython/cpython/Lib/test/test_selectors.py", line 539, in test_register_bad_fd
2020-08-10T12:01:40.0753630Z     bad_f = support.make_bad_fd()
2020-08-10T12:01:40.0757650Z AttributeError: module 'test.support' has no attribute 'make_bad_fd'

Hai Shi: Would you mind to investigate this issue?