Replace JSON serialization with cloudpickle and make reference semantics explicit.

[tbenthompson]

See #138 for older discussion.

Copied from the changelog:

• replaced JSON serialization with cloudpickle. This allows extracting a much
  wider range of objects from the notebook subprocess.
• Reference semantics have changed.
  • Old behavior of tb.get(name) and tb[name]:
    • a reference would be returned for non-JSON-serializable objects.
    • a value would be returned for JSON-serializable objects.
  • Old behavior of tb.ref(name) was identical to tb.get(name).
  • However, now almost all objects are serializable and as a result, under
    the old semantics, a reference would almost never be returned. Therefore,
    when a reference is desired, we now require explicitly requesting a
    reference. The new behavior of tb.get(name) and tb[name] is to always
    return the deserialized object and to never return a reference. The new
    behavior of tb.ref(name) is to always return a reference.

I think this is a substantial improvement because:

1. this PR allows serializing a much wider range of objects
2. makes the API substantially clearer about when a reference will or will not be returned.

I understand that this is a mildly breaking change and adds a dependency and that must be weighed against the benefits. I would argue that the change is very worthwhile!

In my case, I wanted to be able to extract pandas DataFrames and various custom objects from a notebook under test.

[codecov-commenter]

Codecov Report

Merging #153 (0b65427) into main (62d7bd9) will increase coverage by 6.93%.
The diff coverage is 88.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
+ Coverage   91.13%   98.07%   +6.93%     
==========================================
  Files           7        8       +1     
  Lines         361      363       +2     
==========================================
+ Hits          329      356      +27     
+ Misses         32        7      -25