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

Lib/configparser.py

@@ -690,6 +690,7 @@ class RawConfigParser(MutableMapping):
         """
         if isinstance(filenames, (str, bytes, os.PathLike)):
             filenames = [filenames]
+        encoding = io.text_encoding(encoding)
         read_ok = []
         for filename in filenames:
             try: