bpo-13153: Use OS native encoding for converting between Python and Tcl. (GH-16545)

bpo-13153: Use OS native encoding for converting between Python and Tcl. (GH-16545)

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 06cb94bc8419b9a24df6b0d724fcd8e40c6971d6)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>