Support Python 3.7

[dkfellows]

This is how to do builds using Python 3.7 on Travis.

It doesn't work. @Christian-B was too clever by half in some of his classes. 😉

[coveralls]

Pull Request Test Coverage Report for Build 683

• 43 of 47 (91.49%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 91.805%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
spinn_utilities/ranged/abstract_list.py	22	24	91.67%
spinn_utilities/ranged/abstract_sized.py	6	8	75.0%

Totals
Change from base Build 675:	0.02%
Covered Lines:	1994
Relevant Lines:	2172

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💛 - Coveralls

[Christian-B]

Looks like I fixed the key issues so once green can be merged.

Still recommend everyone looks at the files changed to understand the 3.7 monsters.

[dkfellows]

Looks OK to me.

Next, we need to get the rest of the stack also tested with 3.7…