Replace JSON serialization with cloudpickle and make reference semantics explicit.

[tbenthompson]

See https://github.com/nteract/testbook/issues/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

```diff @@ 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 ```