Add property returncode to WSClient

[oz123]

This will be familiar for Python users as subprocess.Popen has
the same attribute. Also, the behavior is such the returncode returns
a None value if the process was not run yet.

Other than that, when the process exists with a numerical code this will
be the value of the return code. If the command executed successfully
the return value will be 0.

[codecov-io]

Codecov Report

Merging #160 into master will decrease coverage by 83.59%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #160      +/-   ##
=========================================
- Coverage   93.56%   9.97%   -83.6%     
=========================================
  Files          13      13              
  Lines        1384    1384              
=========================================
- Hits         1295     138    -1157     
- Misses         89    1246    +1157

Impacted Files	Coverage Δ
config/dateutil_test.py	0% <0%> (-100%)	⬇️
watch/watch_test.py	0% <0%> (-98.71%)	⬇️
config/exec_provider_test.py	0% <0%> (-98.39%)	⬇️
config/incluster_config_test.py	0% <0%> (-97.3%)	⬇️
config/kube_config_test.py	0% <0%> (-95.2%)	⬇️
watch/watch.py	20.48% <0%> (-79.52%)	⬇️
config/kube_config.py	20% <0%> (-68.16%)	⬇️
config/exec_provider.py	17.39% <0%> (-65.22%)	⬇️
config/dateutil.py	35.41% <0%> (-60.42%)	⬇️
config/incluster_config.py	29.78% <0%> (-55.32%)	⬇️

---

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[roycaihw]

/assign

[roycaihw]

/lgtm
/approve

Thanks for having it e2e tested in kubernetes-client/python#956!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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