Bugfix in Excel interface to avoid closing previously open Excel windows

[blnicho]

Fixes Pyomo/pyomo#355

Summary/Motivation:

Use DispatchEx instead of Dispatch to avoid interfering with open Excel windows. This solution was found based on this StackOverflow question: https://stackoverflow.com/questions/18648933/using-pywin32-what-is-the-difference-between-dispatch-and-dispatchex

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[jsiirola]

@blnicho: did you test to confirm that this resolves Pyomo/pyomo#355?

[coveralls]

Coverage remained the same at 62.328% when pulling fc6fef5 on excel_fix into fa6ce97 on master.

[blnicho]

I did some limited testing on my Windows machine using Anaconda and Python 3.7.1 and it appears to resolve the issue. I'm not sure how to add a test for this though.

[jsiirola]

I am a little disconcerted by the comment in the SO post that "DispatchEx is not documented, so you should just use Dispatch"; however, this seems like a good solution to a fundamental problem.

Would you consider adding a comment pointing to some subset of the pyomo issue / this PR / the SO post as a breadcrumb in case the undocumented API ever disappears?

[blnicho]

Yeah I can do that

[codecov-io]

Codecov Report

Merging #59 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   63.13%   63.13%           
=======================================
  Files          87       87           
  Lines        8702     8702           
=======================================
  Hits         5494     5494           
  Misses       3208     3208

Impacted Files	Coverage Δ
pyutilib/excel/spreadsheet_win32com.py	27.56% <100%> (ø)	⬆️

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