Remove all use of OrderedDict

[ianthomas23]

Fixes #1238.

OrderedDict is no longer required as dict has been in insertion order since Python 3.7.

[codecov[bot]]

Codecov Report

Merging #1242 (b06f922) into main (30dda0b) will decrease coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1242      +/-   ##
==========================================
- Coverage   83.37%   83.36%   -0.02%     
==========================================
  Files          35       35              
  Lines        8832     8826       -6     
==========================================
- Hits         7364     7358       -6     
  Misses       1468     1468              

Impacted Files	Coverage Δ
datashader/data_libraries/pandas.py	100.00% <ø> (ø)
datashader/mpl_ext.py	53.40% <ø> (-0.18%)	:arrow_down:
datashader/transfer_functions/__init__.py	87.10% <ø> (-0.03%)	:arrow_down:
datashader/core.py	88.25% <100.00%> (-0.03%)	:arrow_down:
datashader/data_libraries/dask.py	92.80% <100.00%> (-0.06%)	:arrow_down:
datashader/data_libraries/dask_xarray.py	98.94% <100.00%> (-0.01%)	:arrow_down:

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