Fix string comparison: use `==` instead of `is`

[johnomotani]

Can cause a bug on some systems.

[codecov-commenter]

Codecov Report

Merging #267 (5771fe7) into master (02118ed) will increase coverage by 1.58%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   72.93%   74.51%   +1.58%     
==========================================
  Files          13       15       +2     
  Lines        2557     2959     +402     
  Branches      619      721     +102     
==========================================
+ Hits         1865     2205     +340     
- Misses        463      492      +29     
- Partials      229      262      +33     

Impacted Files	Coverage Δ
xbout/plotting/plotfuncs.py	48.71% <60.00%> (+0.44%)	:arrow_up:
xbout/plotting/animate.py	45.37% <100.00%> (ø)
xbout/__init__.py	70.58% <0.00%> (ø)
xbout/utils.py	85.15% <0.00%> (ø)

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[johnomotani]

Can we get a release with this soon?

Yes, planning one before the workshop starts... Hoping to merge some PRs and get animatplot-ng in first!