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/subprocess.py

@@ -693,7 +693,7 @@ def _use_posix_spawn():
 _USE_POSIX_SPAWN = _use_posix_spawn()
 
 
-class Popen(object):
+class Popen:
     """ Execute a child program in a new process.
 
     For a complete description of the arguments see the Python documentation.
@@ -844,6 +844,13 @@ class Popen(object):
 
         self.text_mode = encoding or errors or text or universal_newlines
 
+        # PEP 597: We suppress the EncodingWarning in subprocess module
+        # for now (at Python 3.10), because we focus on files for now.
+        # This will be changed to encoding = io.text_encoding(encoding)
+        # in the future.
+        if self.text_mode and encoding is None:
+            self.encoding = encoding = "locale"
+
         # How long to resume waiting on a child after the first ^C.
         # There is no right value for this.  The purpose is to be polite
         # yet remain good for interactive users trying to exit a tool.