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/pathlib.py

@@ -1241,6 +1241,8 @@ class Path(PurePath):
         Open the file pointed by this path and return a file object, as
         the built-in open() function does.
         """
+        if "b" not in mode:
+            encoding = io.text_encoding(encoding)
         return io.open(self, mode, buffering, encoding, errors, newline,
                        opener=self._opener)
 
@@ -1255,6 +1257,7 @@ class Path(PurePath):
         """
         Open the file in text mode, read it, and close the file.
         """
+        encoding = io.text_encoding(encoding)
         with self.open(mode='r', encoding=encoding, errors=errors) as f:
             return f.read()
 
@@ -1274,6 +1277,7 @@ class Path(PurePath):
         if not isinstance(data, str):
             raise TypeError('data must be str, not %s' %
                             data.__class__.__name__)
+        encoding = io.text_encoding(encoding)
         with self.open(mode='w', encoding=encoding, errors=errors, newline=newline) as f:
             return f.write(data)