[BRANCH-0.4.x] ApiModule can be vendored in the pytest package

[HyukjinKwon]

This PR backports #135 to fix the test failure.

[codecov-io]

Codecov Report

Merging #151 into 0.4.x will increase coverage by 4.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            0.4.x     #151      +/-   ##
==========================================
+ Coverage   83.99%   88.65%   +4.66%     
==========================================
  Files           2        1       -1     
  Lines         531      776     +245     
  Branches       96      183      +87     
==========================================
+ Hits          446      688     +242     
- Misses         63       67       +4     
+ Partials       22       21       -1

Impacted Files	Coverage Δ
cloudpickle/__init__.py
cloudpickle/cloudpickle.py	88.65% <0%> (+4.75%)	⬆️

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