$ ./python -m test -vuall test_c_locale_coercion
...

======================================================================
FAIL: test_external_target_locale_configuration (test.test_c_locale_coercion.LocaleConfigurationTests) (env_var='LC_CTYPE', configured_locale='UTF-8')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/serhiy/py/cpython3.7/Lib/test/test_c_locale_coercion.py", line 254, in test_external_target_locale_configuration
    coercion_expected=False)
  File "/home/serhiy/py/cpython3.7/Lib/test/test_c_locale_coercion.py", line 209, in _check_child_encoding_details
    self.assertEqual(encoding_details, expected_details)
AssertionError: {'fsencoding': 'ascii', 'stdin_info': 'ascii:surrogateesc[128 chars]: ''} != {'fsencoding': 'utf-8', 'stdin_info': 'utf-8:surrogateesc[128 chars]: ''}
- {'fsencoding': 'ascii',
?                 ^^^^^

+ {'fsencoding': 'utf-8',
?                 ^^^^^

   'lang': '',
   'lc_all': '',
   'lc_ctype': 'utf-8',
-  'stderr_info': 'ascii:backslashreplace',
?                  ^^^^^

+  'stderr_info': 'utf-8:backslashreplace',
?                  ^^^^^

-  'stdin_info': 'ascii:surrogateescape',
?                 ^^^^^

+  'stdin_info': 'utf-8:surrogateescape',
?                 ^^^^^

-  'stdout_info': 'ascii:surrogateescape'}
?                  ^^^^^

+  'stdout_info': 'utf-8:surrogateescape'}
?                  ^^^^^


----------------------------------------------------------------------

$ locale   
LANG=
LC_COLLATE="C"
LC_CTYPE="C"
LC_MONETARY="C"
LC_NUMERIC="C"
LC_TIME="C"
LC_MESSAGES="C"
LC_ALL=

Similar issue is issue30836.