Make package Python 2/3 compatible and fix tests

[apacha]

PR 1/2:

• Ran futurizer to make the code-base compatible with Python 2 and 3.
• Fixed unit-tests that currently caused the CI to fail.

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.23%.
The diff coverage is 17.3%.

@@            Coverage Diff            @@
##           master     #12      +/-   ##
=========================================
+ Coverage    6.78%   7.02%   +0.23%     
=========================================
  Files          11      11              
  Lines        3034    3061      +27     
=========================================
+ Hits          206     215       +9     
- Misses       2828    2846      +18

Impacted Files	Coverage Δ
muscima/cropobject_class.py	0% <0%> (ø)	⬆️
muscima/inference.py	0% <0%> (ø)	⬆️
muscima/graph.py	0% <0%> (ø)	⬆️
muscima/dataset.py	0% <0%> (ø)	⬆️
muscima/io.py	0% <0%> (ø)	⬆️
muscima/grammar.py	0% <0%> (ø)	⬆️
muscima/stafflines.py	0% <0%> (ø)	⬆️
muscima/inference_engine_constants.py	74.6% <100%> (+0.83%)	⬆️
muscima/cropobject.py	25.12% <62.5%> (+0.62%)	⬆️
muscima/utils.py	21.05% <66.66%> (+2.87%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 15

• 9 of 52 (17.31%) changed or added relevant lines in 10 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 7.024%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
muscima/utils.py	2	3	66.67%
muscima/graph.py	0	2	0.0%
muscima/cropobject.py	5	8	62.5%
muscima/grammar.py	0	3	0.0%
muscima/io.py	0	3	0.0%
muscima/dataset.py	0	4	0.0%
muscima/cropobject_class.py	0	5	0.0%
muscima/stafflines.py	0	7	0.0%
muscima/inference.py	0	15	0.0%

Files with Coverage Reduction	New Missed Lines	%
muscima/inference.py	1	0.0%
muscima/cropobject_class.py	1	0.0%

Totals
Change from base Build 13:	0.2%
Covered Lines:	215
Relevant Lines:	3061

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💛 - Coveralls