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1. In Tcl 8.2 and later, use Tcl_NewUnicodeObj() when passing a Python
Unicode object rather than going through UTF-8.  (This function
doesn't exist in Tcl 8.1, so there the original UTF-8 code is still
used; in Tcl 8.0 there is no support for Unicode.)  This assumes that
Tcl_UniChar is the same thing as Py_UNICODE; a run-time error is
issued if this is not the case.

2. In Tcl 8.1 and later (i.e., whenever Tcl supports Unicode), when a
string returned from Tcl contains bytes with the top bit set, we
assume it is encoded in UTF-8, and decode it into a Unicode string
object.

Notes:

- Passing Unicode strings to Tcl 8.0 does not do the right thing; this
isn't worth fixing.

- When passing an 8-bit string to Tcl 8.1 or later that has bytes with
the top bit set, Tcl tries to interpret it as UTF-8; it seems to fall
back on Latin-1 for non-UTF-8 bytes.  I'm not sure what to do about
this besides telling the user to disambiguate such strings by
converting them to Unicode (forcing the user to be explicit about the
encoding).

- Obviously it won't be possible to get binary data out of Tk this
way.  Do we need that ability?  How to do it?