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
diff --git a/Lib/io.py b/Lib/io.py
index fbce6ef..01f1df8 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -54,7 +54,7 @@
from _io import (DEFAULT_BUFFER_SIZE, BlockingIOError, UnsupportedOperation,
open, open_code, FileIO, BytesIO, StringIO, BufferedReader,
BufferedWriter, BufferedRWPair, BufferedRandom,
- IncrementalNewlineDecoder, TextIOWrapper)
+ IncrementalNewlineDecoder, text_encoding, TextIOWrapper)
OpenWrapper = _io.open # for compatibility with _pyio