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AppVeyor failed on the backport to Python 3.7 of my fix: PR 12108.
Ok, now I understand the bug in Python 3.7. locale.getlocale(locale.LC_CTYPE)[1] returns None because Python doesn't set LC_CTYPE to the user preferred locale. I'm not sure of which locale is used in practice in that case, but at least I can say that None is not the expected encoding name... str.encode() and bytes.decode() use UTF-8 when None is passed as the encoding. locale.getpreferredencoding() returns 'cp1252' which is the ANSI code page.
Python 3.8 is different. In bpo-34485, I modified Python 3.8 to set LC_CTYPE locale to the user preference (ANSI code page):
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commit 177d921c8c03d30daa32994362023f777624b10d
Author: Victor Stinner <vstinner@redhat.com>
Date: Wed Aug 29 11:25:15 2018 +0200
 bpo-34485, Windows: LC_CTYPE set to user preference (GH-8988)
 
 On Windows, the LC_CTYPE is now set to the user preferred locale at
 startup: _Py_SetLocaleFromEnv(LC_CTYPE) is now called during the
 Python initialization. Previously, the LC_CTYPE locale was "C" at
 startup, but changed when calling setlocale(LC_CTYPE, "") or
 setlocale(LC_ALL, "").
 
 pymain_read_conf() now also calls _Py_SetLocaleFromEnv(LC_CTYPE) to
 behave as _Py_InitializeCore(). Moreover, it doesn't save/restore the
 LC_ALL anymore.
 
 On Windows, standard streams like sys.stdout now always use
 surrogateescape error handler by default (ignore the locale).
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