Implement check_cycle in __getstate__ and __setstate__. Expand python copy tests.

[lukepmccombs]

Adds check_cycle to __getstate__ and __setstate__ so that python copy and deepcopy should be exact duplicates of the original objects.
Expands the digraph copy and deepcopy tests to include check_cycle tests and python object instance tests.

Should resolve #836.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 4463400819

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 97.157%

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Totals
Change from base Build 4417477467:	0.02%
Covered Lines:	14048
Relevant Lines:	14459

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💛 - Coveralls

[IvanIsCoding]

LGTM, we just need to add the release notes for the bugfix and it should be good to go