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/test/test_sys.py b/Lib/test/test_sys.py
index fca05e6..5b004c2 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -591,7 +591,8 @@ def test_sys_flags(self):
"inspect", "interactive", "optimize",
"dont_write_bytecode", "no_user_site", "no_site",
"ignore_environment", "verbose", "bytes_warning", "quiet",
- "hash_randomization", "isolated", "dev_mode", "utf8_mode")
+ "hash_randomization", "isolated", "dev_mode", "utf8_mode",
+ "warn_default_encoding")
for attr in attrs:
self.assertTrue(hasattr(sys.flags, attr), attr)
attr_type = bool if attr == "dev_mode" else int