bpo-43510: Implement PEP 597 opt-in EncodingWarning. (GH-19481)

See [PEP 597](https://www.python.org/dev/peps/pep-0597/).

* Add `-X warn_default_encoding` and `PYTHONWARNDEFAULTENCODING`.
* Add EncodingWarning
* Add io.text_encoding()
* open(), TextIOWrapper() emits EncodingWarning when encoding is omitted and warn_default_encoding is enabled.
* _pyio.TextIOWrapper() uses UTF-8 as fallback default encoding used when failed to import locale module. (used during building Python)
* bz2, configparser, gzip, lzma, pathlib, tempfile modules use io.text_encoding().
* What's new entry

Python/initconfig.c

@@ -94,6 +94,7 @@ static const char usage_3[] = "\
              otherwise activate automatically)\n\
          -X pycache_prefix=PATH: enable writing .pyc files to a parallel tree rooted at the\n\
              given directory instead of to the code tree\n\
+         -X warn_default_encoding: enable opt-in EncodingWarning for 'encoding=None'\n\
 \n\
 --check-hash-based-pycs always|default|never:\n\
     control how Python invalidates hash-based .pyc files\n\
@@ -129,7 +130,8 @@ static const char usage_6[] =
 "PYTHONBREAKPOINT: if this variable is set to 0, it disables the default\n"
 "   debugger. It can be set to the callable of your debugger of choice.\n"
 "PYTHONDEVMODE: enable the development mode.\n"
-"PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.\n";
+"PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.\n"
+"PYTHONWARNDEFAULTENCODING: enable opt-in EncodingWarning for 'encoding=None'.\n";
 
 #if defined(MS_WINDOWS)
 #  define PYTHONHOMEHELP "<prefix>\\python{major}{minor}"
@@ -600,6 +602,7 @@ config_check_consistency(const PyConfig *config)
     assert(config->malloc_stats >= 0);
     assert(config->site_import >= 0);
     assert(config->bytes_warning >= 0);
+    assert(config->warn_default_encoding >= 0);
     assert(config->inspect >= 0);
     assert(config->interactive >= 0);
     assert(config->optimization_level >= 0);
@@ -698,6 +701,7 @@ _PyConfig_InitCompatConfig(PyConfig *config)
     config->parse_argv = 0;
     config->site_import = -1;
     config->bytes_warning = -1;
+    config->warn_default_encoding = 0;
     config->inspect = -1;
     config->interactive = -1;
     config->optimization_level = -1;
@@ -906,6 +910,7 @@ _PyConfig_Copy(PyConfig *config, const PyConfig *config2)
 
     COPY_ATTR(site_import);
     COPY_ATTR(bytes_warning);
+    COPY_ATTR(warn_default_encoding);
     COPY_ATTR(inspect);
     COPY_ATTR(interactive);
     COPY_ATTR(optimization_level);
@@ -1007,6 +1012,7 @@ _PyConfig_AsDict(const PyConfig *config)
     SET_ITEM_WSTR(platlibdir);
     SET_ITEM_INT(site_import);
     SET_ITEM_INT(bytes_warning);
+    SET_ITEM_INT(warn_default_encoding);
     SET_ITEM_INT(inspect);
     SET_ITEM_INT(interactive);
     SET_ITEM_INT(optimization_level);
@@ -1271,6 +1277,7 @@ _PyConfig_FromDict(PyConfig *config, PyObject *dict)
     GET_WSTRLIST(warnoptions);
     GET_UINT(site_import);
     GET_UINT(bytes_warning);
+    GET_UINT(warn_default_encoding);
     GET_UINT(inspect);
     GET_UINT(interactive);
     GET_UINT(optimization_level);

Python/preconfig.c

@@ -169,6 +169,7 @@ _PyPreCmdline_SetConfig(const _PyPreCmdline *cmdline, PyConfig *config)
     COPY_ATTR(isolated);
     COPY_ATTR(use_environment);
     COPY_ATTR(dev_mode);
+    COPY_ATTR(warn_default_encoding);
     return _PyStatus_OK();
 
 #undef COPY_ATTR
@@ -257,9 +258,17 @@ _PyPreCmdline_Read(_PyPreCmdline *cmdline, const PyPreConfig *preconfig)
         cmdline->dev_mode = 0;
     }
 
+    // warn_default_encoding
+    if (_Py_get_xoption(&cmdline->xoptions, L"warn_default_encoding")
+            || _Py_GetEnv(cmdline->use_environment, "PYTHONWARNDEFAULTENCODING"))
+    {
+        cmdline->warn_default_encoding = 1;
+    }
+
     assert(cmdline->use_environment >= 0);
     assert(cmdline->isolated >= 0);
     assert(cmdline->dev_mode >= 0);
+    assert(cmdline->warn_default_encoding >= 0);
 
     return _PyStatus_OK();
 }

Python/sysmodule.c

@@ -2514,6 +2514,7 @@ static PyStructSequence_Field flags_fields[] = {
     {"isolated",                "-I"},
     {"dev_mode",                "-X dev"},
     {"utf8_mode",               "-X utf8"},
+    {"warn_default_encoding",   "-X warn_default_encoding"},
     {0}
 };
 
@@ -2521,7 +2522,7 @@ static PyStructSequence_Desc flags_desc = {
     "sys.flags",        /* name */
     flags__doc__,       /* doc */
     flags_fields,       /* fields */
-    15
+    16
 };
 
 static int
@@ -2560,6 +2561,7 @@ set_flags_from_config(PyInterpreterState *interp, PyObject *flags)
     SetFlag(config->isolated);
     SetFlagObj(PyBool_FromLong(config->dev_mode));
     SetFlag(preconfig->utf8_mode);
+    SetFlag(config->warn_default_encoding);
 #undef SetFlagObj
 #undef SetFlag
     return 0;