python: Make invalid UTF-8 sequence messages consistent across Python versions.

python: Make invalid UTF-8 sequence messages consistent across Python versions.

Given the invalid input <C0 22>, some versions of Python report <C0> as the
invalid sequence and other versions report <C0 22> as the invalid sequence.
Similarly, given input <ED 80 7F>, some report <ED 80> and others report
<ED 80 7F> as the invalid sequence.  This caused spurious test failures for
the test "no invalid UTF-8 sequences in strings - Python", so this commit
makes the messages consistent by dropping the extra trailing byte from the
message.

I first noticed the longer sequences <C0 22> and <ED 80 7F> on Ubuntu
10.04 with python version 2.6.5-0ubuntu1, but undoubtedly it exists
elsewhere also.