bpo-34589: Make _PyCoreConfig.coerce_c_locale private (GH-9371)

_PyCoreConfig:

* Rename coerce_c_locale to _coerce_c_locale
* Rename coerce_c_locale_warn to _coerce_c_locale_warn

These fields are now private (name prefixed by "_").
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 80233a5..c00000e 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -277,8 +277,6 @@
         'filesystem_errors': None,
 
         'utf8_mode': 0,
-        'coerce_c_locale': 0,
-        'coerce_c_locale_warn': 0,
 
         'pycache_prefix': NULL_STR,
         'program_name': './_testembed',
@@ -306,6 +304,8 @@
         '_install_importlib': 1,
         '_check_hash_pycs_mode': 'default',
         '_frozen': 0,
+        '_coerce_c_locale': 0,
+        '_coerce_c_locale_warn': 0,
     }
 
     def get_stdio_encoding(self, env):