♻️ comply with new pandas index handling

[jvdd]

This PR solves issue #36

Note: I benchmarked .is_integer vs the current isinstance checking. My findings are that .is_integer is slightly slower, but this is a constant (minor) difference, meaning that just like for type checking the runtime does not deteriorate for longer index lengths.

[codecov-commenter]

Codecov Report

Merging #37 (3826dd7) into main (05b252c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files           6        6           
  Lines         389      389           
=======================================
  Hits          385      385           
  Misses          4        4           

Impacted Files	Coverage Δ
plotly_resampler/figure_resampler.py	98.80% <100.00%> (ø)

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[jonasvdd]

LGTM!