We used to convert the unicode object directly to a string in the server
encoding by calling Python's PyUnicode_AsEncodedString function. In other
words, we used Python's routines to do the encoding. However, that has a
few problems. First of all, it required keeping a mapping table of Python
encoding names and PostgreSQL encodings. But the real killer was that Python
doesn't support EUC_TW and MULE_INTERNAL encodings at all.

Instead, convert the Python unicode object to UTF-8, and use PostgreSQL's
encoding conversion functions to convert from UTF-8 to server encoding. We
were already doing the same in the other direction in PLyUnicode_FromString,
so this is more consistent, too.

Note: This makes SQL_ASCII to behave more leniently. We used to map
SQL_ASCII to Python's 'ascii', which on Python means strict 7-bit ASCII
only, so you got an error if the python string contained anything but pure
ASCII. You no longer get an error; you get the UTF-8 representation of the
string instead.

Backpatch to 9.0, where these conversions were introduced.

Jan Urbański

This will (hopefully) eliminate the need for the
plpython_unicode_0.out expected file.

It is redundant, given the error context.

Jan Urbański

Use the built-in TypeError, not SPIError, for errors having to do with
argument counts or types.  Use SPIError, not simply plpy.Error, for
errors in PLy_spi_execute_plan.  Finally, do not set a Python
exception if PyArg_ParseTuple failed, as it already sets the correct
exception.

Jan Urbański

The temporarily broken plpython_unicode test shows a case where this
is used.

Do remaining fix-ups on the expected files at the same time.

It's not clear to me what should happen to the other plpython_unicode
variant expected files, but this patch gets things passing on my own
machines and at least some of the buildfarm.

Mimic the Python interpreter's own logic for printing exceptions instead
of just using the straight str() call, so that
you get

    plpy.SPIError

instead of

    <class 'plpy.SPIError'>

and for built-in exceptions merely

    UnicodeEncodeError

Besides looking better this cuts down on the endless version differences
in the regression test expected files.