Find a more reliable source origin for diagnostic

[stevefan1999]

Coming from m-labs/artiq#1671:

It seems like one of the issue stems from here:

pythonparser/pythonparser/diagnostic.py

Lines 51 to 53 in 5413ee5

	if len(set(map(lambda x: x.source_buffer,
	[location] + highlights))) > 1:
	raise ValueError("location and highlights must refer to the same source buffer")

And indeed the evaluation of the expression shows there are more than 2 elements.

While the source is clearly different, and it kind of makes sense for the compiler to throw an error like this, shouldn't we find a better source of determining the source origin?