igraph py2 compatibility

[xrotwang]

igraph seems to use "real" python attributes to store additional data for
edges and vertices. Thus, it requires the attribute names to be ASCII encoded
rather than unicode strings in python 2.

[codecov-io]

Codecov Report

Merging #302 into master will decrease coverage by <.01%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
- Coverage   90.46%   90.46%   -0.01%     
==========================================
  Files         119      119              
  Lines       15817    15819       +2     
==========================================
+ Hits        14309    14310       +1     
- Misses       1508     1509       +1

Impacted Files	Coverage Δ
lingpy/convert/graph.py	82.51% <25%> (-0.92%)	⬇️
lingpy/compare/phylogeny.py	82.62% <0%> (+0.05%)	⬆️

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