On Windows use UTF-16 (or UTF-32 for 32-bit Tcl_UniChar) with the
"surrogatepass" error handler for converting to/from Tcl Unicode objects.
On Linux use UTF-8 with the "surrogateescape" error handler for converting
to/from Tcl String objects.
Converting strings from Tcl to Python and back now never fails
(except MemoryError).
(cherry picked from commit 06cb94bc84)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
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