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

1 files changed, 3 insertions, 1 deletions

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 686b6ca..54d70ef 100644
--- a/Python/sysmodule.c
+++ b/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;