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/test/test_pickle.py

@@ -483,7 +483,8 @@ class CompatPickleTests(unittest.TestCase):
                 if exc in (BlockingIOError,
                            ResourceWarning,
                            StopAsyncIteration,
-                           RecursionError):
+                           RecursionError,
+                           EncodingWarning):
                     continue
                 if exc is not OSError and issubclass(exc, OSError):
                     self.assertEqual(reverse_mapping('builtins', name),