Support nwb_version as a fixed-length string

[oruebel]

Motivation

Fix #1668

In #1612 we added support to do version checks on NWB HDF5 files to improve error reporting. The new nwb_version function implictly assumed that the attribute was written as a variable-lenght string. This leads to errors for NWB files that may use fixed-length strings instead (see #1668 for detail). This PR updated the NWBHDF5IO.new_version property to make sure the resulting bytes object is being decoded to a string in case that the attribute was written as a fixed-length bytes.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1669 (65aebc4) into dev (bdb94a7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1669   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
=======================================
  Files          26       26           
  Lines        2585     2587    +2     
  Branches      496      497    +1     
=======================================
+ Hits         2378     2380    +2     
  Misses        132      132           
  Partials       75       75           

Flag	Coverage Δ
integration	71.55% <100.00%> (+0.02%)	⬆️
unit	83.61% <0.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
src/pynwb/__init__.py	78.94% <100.00%> (+0.28%)	⬆️

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