Add support for python 3.7

[yakky]

Only travis and tox configuration changes was needed

[codecov]

Codecov Report

Merging #87 into master will decrease coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   96.28%   96.05%   -0.23%     
==========================================
  Files           7        7              
  Lines         888      888              
  Branches       71       71              
==========================================
- Hits          855      853       -2     
- Misses         20       21       +1     
- Partials       13       14       +1

Impacted Files	Coverage Δ
taiga/client.py	90.29% <0%> (-1.95%)	⬇️

---

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[codecov]

Codecov Report

Merging #87 into master will decrease coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   96.28%   96.05%   -0.23%     
==========================================
  Files           7        7              
  Lines         888      888              
  Branches       71       71              
==========================================
- Hits          855      853       -2     
- Misses         20       21       +1     
- Partials       13       14       +1

Impacted Files	Coverage Δ
taiga/client.py	90.29% <0%> (-1.95%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8c52977...003abd3. Read the comment docs.

[coveralls]

Coverage increased (+1.2%) to 97.651% when pulling 003abd3 on feature/python-3.7 into 8c52977 on master.

[morlandi]

@yakky ... is Python 3.7 really supported by Travis on Github ?
What a news !