Add python 3.5 support

[lewisjared]

Fixes #18

[codecov]

Codecov Report

Merging #20 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   98.83%   98.82%   -0.02%     
==========================================
  Files           7        7              
  Lines         775      767       -8     
  Branches      152      152              
==========================================
- Hits          766      758       -8     
  Misses          9        9

Impacted Files	Coverage Δ
src/scmdata/time.py	100% <ø> (ø)	⬆️
src/scmdata/units.py	100% <100%> (ø)	⬆️
src/scmdata/dataframe.py	99.74% <100%> (-0.01%)	⬇️

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[znicholls]

duck typing for life