RF: Replace pickle with cloudpickle for input/output specs

[effigies]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

Replace pickle with cloudpickle for input/output specs.

As noted in #96 (comment), the problem with cloudpickling specs should have been resolved by cloudpipe/cloudpickle#245. This hit version 0.8.0, so I'm updating the minimum required version, as well.

Checklist

• All tests passing
• I have added tests to cover my changes
• I have updated documentation (if necessary)
• My code follows the code style of this project
  (we are using black: you can pip install pre-commit,
  run pre-commit install in the pydra directory
  and black will be run automatically with each commit)

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #104 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #104      +/-   ##
=========================================
- Coverage    86.5%   86.5%   -0.01%     
=========================================
  Files          14      14              
  Lines        2179    2178       -1     
  Branches      545     545              
=========================================
- Hits         1885    1884       -1     
  Misses        205     205              
  Partials       89      89

Flag	Coverage Δ
#unittests	86.5% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pydra/engine/core.py	87.39% <100%> (-0.04%)	⬇️

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