Fix code which triggered DeprecationWarnings

[jwiggins]

Fixes #339

[jwiggins]

Note that piltostring is just a wrapper around Image.tobytes (or Image.tostring when not using Pillow). All of the fromstring -> frombuffer conversions are passing the output of piltostring.

[codecov-io]

Codecov Report

Merging #343 into master will not change coverage.
The diff coverage is 14.28%.

@@           Coverage Diff           @@
##           master     #343   +/-   ##
=======================================
  Coverage   33.94%   33.94%           
=======================================
  Files         206      206           
  Lines       18290    18290           
  Branches     2414     2414           
=======================================
  Hits         6209     6209           
  Misses      11688    11688           
  Partials      393      393

Impacted Files	Coverage Δ
enable/component.py	55.76% <0%> (ø)	⬆️
enable/savage/svg/document.py	27.04% <0%> (ø)	⬆️
enable/scrolled.py	23.52% <0%> (ø)	⬆️
enable/tools/viewport_zoom_tool.py	49.54% <0%> (ø)	⬆️
enable/container.py	53.72% <100%> (ø)	⬆️

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[jvkersch]

LGTM