add test case for issue #71

[burnpanck]

Make sure PintArray is pickleable.

• [x] Closes #71 (insert issue number)
• [x] Executed pre-commit run --all-files with no errors
• [x] The change is fully covered by automated unit tests
• [x] Documented in docs/ as appropriate
• [x] Added an entry to the CHANGES file

[andrewgsavage]

nice, thanks for getting this working.