Fix deprecated getargspec calls in Python 3.

[corranwebster]

Same idea as enthought/traitsui#530

[codecov-io]

Codecov Report

Merging #369 into master will increase coverage by <.01%.
The diff coverage is 42.1%.

@@            Coverage Diff             @@
##           master     #369      +/-   ##
==========================================
+ Coverage   36.34%   36.34%   +<.01%     
==========================================
  Files         481      481              
  Lines       26406    26418      +12     
  Branches     3911     3915       +4     
==========================================
+ Hits         9597     9602       +5     
- Misses      16393    16398       +5     
- Partials      416      418       +2

Impacted Files	Coverage Δ
pyface/tree/node_tree.py	0% <0%> (ø)	⬆️
pyface/ui/qt4/action/action_item.py	41.8% <100%> (+0.72%)	⬆️
pyface/wx/drag_and_drop.py	20.61% <33.33%> (+0.29%)	⬆️
pyface/ui/wx/action/action_item.py	36.61% <50%> (-0.04%)	⬇️

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