MAINT: replace istype with isinstance

[Zeroto521]

No description provided.

[codecov]

Codecov Report

Merging #189 (1cb46ee) into master (1d16e44) will decrease coverage by 0.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   99.78%   99.36%   -0.43%     
==========================================
  Files          22       22              
  Lines         476      476              
  Branches       49       49              
==========================================
- Hits          475      473       -2     
- Misses          0        1       +1     
- Partials        1        2       +1     

Impacted Files	Coverage Δ
dtoolkit/geography/buffer.py	100.00% <100.00%> (ø)
dtoolkit/util/_validation.py	77.77% <0.00%> (-22.23%)	⬇️

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