Updated syntax for python3

[duncanmmacleod]

This PR updates any and all problems I could find related to python3 compatibility. The changes are fairly well separated into commits that touch one aspect of compatibility, so should be easy enough to review.

This should mean that the unit tests all run on python3, so I want to promote the travis-ci python3 jobs to a 'must pass'.

cc: @alurban, @macedo22

[coveralls]

Pull Request Test Coverage Report for Build 244

• 21 of 25 (84.0%) changed or added relevant lines in 10 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.6%) to 40.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
gwdetchar/io/html.py	0	1	0.0%
gwdetchar/io/ligolw.py	2	3	66.67%
gwdetchar/omega/plot.py	0	1	0.0%
gwdetchar/tests/test_cli.py	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
gwdetchar/omega/html.py	1	0.0%
gwdetchar/omega/plot.py	1	0.0%

Totals
Change from base Build 242:	0.6%
Covered Lines:	428
Relevant Lines:	1070

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 243

• 17 of 25 (68.0%) changed or added relevant lines in 10 files are covered.
• 6 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.6%) to 40.0%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
gwdetchar/io/html.py	0	1	0.0%
gwdetchar/io/ligolw.py	2	3	66.67%
gwdetchar/omega/plot.py	0	1	0.0%
gwdetchar/tests/test_const.py	3	5	60.0%
gwdetchar/tests/test_cli.py	2	5	40.0%

Files with Coverage Reduction	New Missed Lines	%
gwdetchar/omega/html.py	1	0.0%
gwdetchar/omega/plot.py	1	0.0%
gwdetchar/tests/test_cds.py	2	100.0%
gwdetchar/tests/test_daq.py	2	100.0%

Totals
Change from base Build 242:	0.6%
Covered Lines:	428
Relevant Lines:	1070

---

💛 - Coveralls

[alurban]

This all looks good to me and pipelines are passing, I'm going to merge. One small comment in gwdetchar/omega/html.py, on the old line 31: this import of pytz.timezone was meant to allow you to write the detector's local timezone in the analysis summary (along with UTC), but I never got that to work properly, and I guess I forgot to remove unused imports.