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, 7 insertions, 0 deletions

diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 4b2547c98f..efcf7a7fb3 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -543,6 +543,9 @@ def NamedTemporaryFile(mode='w+b', buffering=-1, encoding=None,
     if _os.name == 'nt' and delete:
         flags |= _os.O_TEMPORARY
 
+    if "b" not in mode:
+        encoding = _io.text_encoding(encoding)
+
     (fd, name) = _mkstemp_inner(dir, prefix, suffix, flags, output_type)
     try:
         file = _io.open(fd, mode, buffering=buffering,
@@ -583,6 +586,9 @@ else:
         """
         global _O_TMPFILE_WORKS
 
+        if "b" not in mode:
+            encoding = _io.text_encoding(encoding)
+
         prefix, suffix, dir, output_type = _sanitize_params(prefix, suffix, dir)
 
         flags = _bin_openflags
@@ -638,6 +644,7 @@ class SpooledTemporaryFile:
         if 'b' in mode:
             self._file = _io.BytesIO()
         else:
+            encoding = _io.text_encoding(encoding)
             self._file = _io.TextIOWrapper(_io.BytesIO(),
                             encoding=encoding, errors=errors,
                             newline=newline)