Do not use warnings from numpy

[danigm]

numpy has removed the warnings symbol from the init.py file so np.warnings is not valid in newer versions of numpy. This patch justs imports the warnings module and use it directly.

[codecov[bot]]

Codecov Report

Merging #1176 (141210c) into main (21a8614) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1176      +/-   ##
==========================================
+ Coverage   85.36%   85.38%   +0.01%     
==========================================
  Files          35       35              
  Lines        8010     8011       +1     
==========================================
+ Hits         6838     6840       +2     
+ Misses       1172     1171       -1     

Impacted Files	Coverage Δ
datashader/core.py	88.25% <100.00%> (+0.20%)	:arrow_up:
datashader/transfer_functions/__init__.py	86.89% <100.00%> (+0.02%)	:arrow_up:

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