Change upload value to contain Python types

[pbugnion]

Prior to this PR, the .value traitlet for the FileUpload widget was a list of dictionaries. Each dictionary contained the raw metadata generated by the browser, as well as the content as a memoryview.

This PR changes the return value to be a tuple of Bunch instances. Attribute names inside the bunch are converted to snake case and the last modified is converted to a zoned datetime.

For instance:

# f is a FileUpload instance
>>> f.value
({'name': 'some-name.png',
  'type': 'image/png',
  'size': 29841,
  'content': <memory at 0x118001940>,
  'last_modified': 1578491179205},)

This PR also removes syncing an 'error' attribute per file. This was not exposed in the kernel, and it was never set to anything apart from an empty string in the frontend. Note that the global error attribute remains.

Note that this PR has some overlap with #2715 : both share datetime serialization / deserialization considerations. In here, I have done the least amount of work -- I just send a timestamp over the wire, and manage conversion in the kernel. Once 2548 is merged, it may be worth overhauling serialization to have a common interface. That won't be a breaking change.

Closes issue #2727 .

[pbugnion]

This is now ready for review.

[jasongrout]

Our general convention, at least in default controls, has been snake_case for the attributes in the protocol messages. Should we keep it snake_case here, as last_modified?

[jasongrout]

Another option is to just call it modified, or even mtime, which mirrors the stat() structure and how you address it in python and presumably lots of other languages that let you make a stat call.

[pbugnion]

mtime in Python is a timestamp, whereas we're returning a datetime. That warrants using a different name, I think.

I'll go withlast_modified, but this is not strongly held (so feel free to change it).

[jasongrout]

This looks great! A couple of inline comments above, and then I think it's ready to go.

[pbugnion]

Thanks for the feedback. I made all the changes discussed.

[jasongrout]

Thanks! This looks really great.