fix for head and tail port

[haisland0909]

I got error when I write edge with headport and tailport option.
The error caused by pygraphviz.
When pygraphviz creates svg file from dot file with headport and tailport option, the title becomes
"source:tailport->target:headport" and EdgeFactory looks it as key of edge dict.

[codecov]

Codecov Report

Merging #34 into master will increase coverage by 0.81%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   76.94%   77.75%   +0.81%     
==========================================
  Files          23       23              
  Lines         590      598       +8     
==========================================
+ Hits          454      465      +11     
+ Misses        136      133       -3     

Impacted Files	Coverage Δ
graphviz2drawio/mx/EdgeFactory.py	100.00% <100.00%> (ø)
graphviz2drawio/mx/MxGraph.py	88.40% <100.00%> (+1.44%)	⬆️
graphviz2drawio/mx/CurveFactory.py	100.00% <0.00%> (+4.34%)	⬆️
graphviz2drawio/mx/GraphObj.py	100.00% <0.00%> (+12.50%)	⬆️

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[hbmartin]

Thank you!