Fix handling of version 2.0b

[rly]

Motivation

Fix #1650. In the history of NWB 2.x files, I have found that only version "2.0b" does not follow the X.Y.Z format of semantic versioning. This patch covers that use case while not making the code terribly complicated.

How to test the behavior?

Run tests

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 #1651 (f3e1751) into dev (1dc9125) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1651      +/-   ##
==========================================
+ Coverage   91.97%   91.99%   +0.01%     
==========================================
  Files          26       26              
  Lines        2581     2585       +4     
  Branches      494      496       +2     
==========================================
+ Hits         2374     2378       +4     
  Misses        132      132              
  Partials       75       75              

Flag	Coverage Δ
integration	71.52% <100.00%> (+0.04%)	⬆️
unit	83.67% <0.00%> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
src/pynwb/io/utils.py	100.00% <100.00%> (ø)

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