switch to python3

[ltalirz]

fix #53 fix #52

[ltalirz]

@yakutovicha @danieleongari Does this solve the issues?

[coveralls]

Pull Request Test Coverage Report for Build 307

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage remained the same at ?%

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Files with Coverage Reduction	New Missed Lines	%
aiida_zeopp/data/parameters.py	1	94.19%
aiida_zeopp/parsers/plain.py	1	88.89%
aiida_zeopp/tests/init.py	1	88.64%
<!--	Total:	3		-->

Totals
Change from base Build 303:	0%
Covered Lines:
Relevant Lines:	0

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 313

• 14 of 14 (100.0%) changed or added relevant lines in 5 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.3%) to 93.228%

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Files with Coverage Reduction	New Missed Lines	%
aiida_zeopp/data/parameters.py	1	93.98%
aiida_zeopp/parsers/plain.py	1	90.32%
<!--	Total:	2		-->

Totals
Change from base Build 303:	0.3%
Covered Lines:	413
Relevant Lines:	443

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💛 - Coveralls