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/Programs/_testembed.c b/Programs/_testembed.c
index 99772ea..1288032 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -330,8 +330,6 @@
 
     printf("filesystem_encoding = %s\n", config->filesystem_encoding);
     printf("filesystem_errors = %s\n", config->filesystem_errors);
-    printf("coerce_c_locale = %i\n", config->coerce_c_locale);
-    printf("coerce_c_locale_warn = %i\n", config->coerce_c_locale_warn);
     printf("utf8_mode = %i\n", config->utf8_mode);
 
     printf("pycache_prefix = %ls\n", config->pycache_prefix);
@@ -385,6 +383,8 @@
     printf("_install_importlib = %i\n", config->_install_importlib);
     printf("_check_hash_pycs_mode = %s\n", config->_check_hash_pycs_mode);
     printf("_frozen = %i\n", config->_frozen);
+    printf("_coerce_c_locale = %i\n", config->_coerce_c_locale);
+    printf("_coerce_c_locale_warn = %i\n", config->_coerce_c_locale_warn);
 
 #undef ASSERT_EQUAL
 #undef ASSERT_STR_EQUAL
@@ -482,7 +482,7 @@
     putenv("PYTHONMALLOCSTATS=0");
     config.malloc_stats = 1;
 
-    /* FIXME: test coerce_c_locale and coerce_c_locale_warn */
+    /* FIXME: test _coerce_c_locale and _coerce_c_locale_warn */
 
     putenv("PYTHONUTF8=0");
     Py_UTF8Mode = 0;
@@ -606,8 +606,8 @@
     /* Test _PyCoreConfig.isolated=1 */
     _PyCoreConfig config = _PyCoreConfig_INIT;
 
-    /* Set coerce_c_locale and utf8_mode to not depend on the locale */
-    config.coerce_c_locale = 0;
+    /* Set _coerce_c_locale and utf8_mode to not depend on the locale */
+    config._coerce_c_locale = 0;
     config.utf8_mode = 0;
     /* Use path starting with "./" avoids a search along the PATH */
     config.program_name = L"./_testembed";