Fixed latex export for svg data in python 3

[MSeal]

Fixes #981

Verified that the rendered latex output matches python 2 output visually. This will probably fix other python exports outside of latex svg given the code path involved in the fix.

[akhmerov]

This is too strict and will break some data types; compare with even a partial listing here: https://jupyter.readthedocs.io/en/latest/reference/mimetype.html

Also this change invalidates the follow-up comment, please keep them in sync.

[MSeal]

Fair point on the strictness.

[akhmerov]

In the original implementation I've relied on the nbformat spec that guaranteed that json data is parsed directly and not serialized.

I don't actually understand why that bit requires changing to support svg: IIUC svg should be of text-type and shouldn't be treated as json by that code.

[MSeal]

If you only add the svg test I added you'll see if fails for python 3 in the same way the original issue describes. The data object coming back from out.data[mime_type] is definitely bytes and adding data = data.decode('utf-8'), or even the original data = json.dumps(data), generically to all byte data before any other processing seemed wrong.

I'll look into the test case I added and see if there's a less strict way to solve the issue.

[akhmerov]

At the very least lists/dicts should be detected as definitely json

[MSeal]

I looked into the failing pattern, it's when the mimetype is application/pdf that it fails in the test case originally.

I changed the implementation to mirror the original behavior more closely, with just logically moving the base64 data type checks in front of the json / text checks. Can you take a look again @akhmerov for any issue you can see?

[MSeal]

Thanks for the review!