Py3 test fix

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extra : transplant_source : %1CI-%A6a%F0%97%A1%AF9%22%C1%03%D7%0C%84%A3%DE%CBc
prepare release of 0.17.3
Added tag 0.17.3 for changeset b0faba6967e7
Include CHANGES.rst into the source distribution
Merge pull request #172 from yarikoptic/upstream-0.17

Include CHANGES.rst into the source distribution
Guard deallocation with PyObject_GC_Untrack, trac #796.
Test for trac #796.
Prep for 0.17.4 release.