FIX: Python 3 compatibility issue while loading multiframe ECAT files

[effigies]

This PR provides a multiframe ECAT file for our test battery. A couple minor cleanups I saw while looking through this code/docs.

@idoimaging If you would like, you can use my branch as a base to commit your change, e.g.,

git remote add effigies https://github.com/effigies/nibabel.git
git fetch effigies
git checkout -t effigies/fix/ecat_py3
<make change>
git add nibabel/ecat.py
git commit -m <commit message>
git push -u origin fix/ecat_py3

Edit: I went ahead and added the fix with credit.

Fixes #776.

[codecov-io]

Codecov Report

Merging #777 into master will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
+ Coverage   89.99%   90.04%   +0.05%     
==========================================
  Files          94       94              
  Lines       12009    12010       +1     
  Branches     2133     2133              
==========================================
+ Hits        10807    10814       +7     
+ Misses        859      851       -8     
- Partials      343      345       +2

Impacted Files	Coverage Δ
nibabel/ecat.py	87.98% <66.66%> (+1.71%)	⬆️
nibabel/cmdline/parrec2nii.py	32.73% <0%> (ø)	⬆️

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[effigies]

@idoimaging Does this resolve your issue?

[effigies]

We'll trust to the tests.