Switch spreadsheet interface to load dependencies on demand

[jsiirola]

Fixes: #N/A

Summary/Motivation:

Reworks how optional dependent libraries are imported so that many optional dependencies (notably pkg_resources and openpyxl) are only imported "on demand". This significantly speeds up importing pyutilib.

Changes proposed in this PR:

• rework optional dependency imports

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[coveralls]

Coverage increased (+0.09%) to 61.12% when pulling f25a549 on jsiirola:defer-excel-imports into f49e95e on PyUtilib:master.

[codecov-io]

Codecov Report

Merging #84 into master will decrease coverage by 0.09%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master      #84     +/-   ##
=========================================
- Coverage   63.06%   62.96%   -0.1%     
=========================================
  Files          87       87             
  Lines        8784     8817     +33     
=========================================
+ Hits         5540     5552     +12     
- Misses       3244     3265     +21

Impacted Files	Coverage Δ
pyutilib/excel/spreadsheet.py	77.77% <75%> (+13.94%)	⬆️
pyutilib/subprocess/processmngr.py	60.34% <0%> (-1.68%)	⬇️
pyutilib/workflow/task.py	97.68% <0%> (-0.57%)	⬇️
pyutilib/dev/runtests.py	71.64% <0%> (-0.54%)	⬇️
pyutilib/misc/config.py	94.28% <0%> (-0.42%)	⬇️
pyutilib/workflow/functor.py	97.51% <0%> (-0.41%)	⬇️
pyutilib/component/core/core.py	84.73% <0%> (-0.33%)	⬇️
pyutilib/misc/archivereader.py	82.01% <0%> (-0.18%)	⬇️
pyutilib/pyro/dispatcher.py	22.93% <0%> (-0.18%)	⬇️

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