PyErr_SetFromErrnoWithFilename(): New function which supports setting

PyErr_SetFromErrnoWithFilename(): New function which supports setting
an exception from errno, with a supplied filename (primarily used by
IOError and OSError).  If class exceptions are used then the exception
is instantiated with a 3-tuple: (errno, strerror, filename).  For
backwards compatibility reasons, if string exceptions are used,
filename is ignored.

PyErr_SetFromErrno(): Implement in terms of
PyErr_SetFromErrnoWithFilename().