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/Lib/bz2.py b/Lib/bz2.py
index ce07ebe..1da3ce6 100644
--- a/Lib/bz2.py
+++ b/Lib/bz2.py
@@ -311,6 +311,7 @@ def open(filename, mode="rb", compresslevel=9,
     binary_file = BZ2File(filename, bz_mode, compresslevel=compresslevel)
 
     if "t" in mode:
+        encoding = io.text_encoding(encoding)
         return io.TextIOWrapper(binary_file, encoding, errors, newline)
     else:
         return binary_file