Patch version 4:
* Handle PYTHONLEGACYWINDOWSFSENCODING: this env var now disables the UTF-8 mode and has the priority over -X utf8 and PYTHONUTF8
* Add an unit test on PYTHONUTF8 env var and -E cmdline option
* Add an unit test on the POSIX locale
* Fix initstdio() to handle correctly empty PYTHONIOENCODING: this bug affects Python 3.6 as well and is not directly related to the PEP 540
* Fix to handle correctly PYTHONUTF8 set to an empty string (ignore it)
* Skip an unit test in test_utf8mode which failed with the POSIX locale
Note: This patch still has the sys.argv encoding bug with locale encodings different than ASCII and UTF-8.