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, 1 insertions, 0 deletions

diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 666c1e4..09f9a29 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -153,6 +153,7 @@ typedef struct PyConfig {
     PyWideStringList warnoptions;
     int site_import;
     int bytes_warning;
+    int warn_default_encoding;
     int inspect;
     int interactive;
     int optimization_level;