Interpolate printf-style logging input to logging function using vsnprintf

On some older Python 2.7 systems on Linux, the PyUnicode_FromFormatV function was mishandling some printf interpolation arguments, in this case "%.*s", a string with a given length.

PyUnicode_FromFormatV should actually just dump the remainder of the format string but in some cases it segfaults instead when it encounters something it doesn't understand.

In addition, we want to be able to log output from these more unusual interpolation strings, so it seems better to use a C function to write to a buffer before sending it to CPython for logging.