Add support for Python 3

[pbauer]

Fixes #80

[coveralls]

Pull Request Test Coverage Report for Build 413

• 8 of 8 (100.0%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 89.354%

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Files with Coverage Reduction	New Missed Lines	%
src/collective/ifttt/browser/content_trigger.py	1	87.04%
<!--	Total:	1		-->

Totals
Change from base Build 401:	0.1%
Covered Lines:	470
Relevant Lines:	526

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💛 - Coveralls

[Shriyanshagro]

Ported to Python3 during PloneConf2018's sprint at Microsoft, Tokyo!!