Support pandas 2.1

[ianthomas23]

Now that there is a SpatialPandas RC the supports Pandas 2.1 (https://pypi.org/project/spatialpandas/0.4.9rc1/#files and https://anaconda.org/pyviz/spatialpandas/files), we can remove the temporary pin of #1275.

[codecov]

Codecov Report

Merging #1276 (c69d941) into main (7f43eda) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1276      +/-   ##
==========================================
+ Coverage   83.53%   83.54%   +0.01%     
==========================================
  Files          35       35              
  Lines        9128     9134       +6     
==========================================
+ Hits         7625     7631       +6     
  Misses       1503     1503              

Files Changed	Coverage Δ
datashader/bundling.py	98.64% <100.00%> (+0.02%)	⬆️

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[ianthomas23]

It wasn't as easy as just removing the pandas pin. There were also problems in the edge bundling code when reading rows from a Pandas DataFrame; previously the code took whatever column order it was given which is not reliable when converting column names to deal with edge_id. Now we are using the recommended df.itertuples() and specifically extracting the columns in a particular order.

All of this code looks ripe for performance improvement eventually.