Remove unneeded future imports

[ianthomas23]

This PR removes unneeded __future__ imports of absolute_import, division and print_function. Support for Python 2.* required them and they are no longer necessary. Table of minimum versions at https://docs.python.org/3/library/__future__.html shows they have all been available since Python 3.0.

This is part of issue #1112.

[codecov[bot]]

Codecov Report

Merging #1115 (81a9846) into master (f09c47c) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1115   +/-   ##
=======================================
  Coverage   85.07%   85.07%           
=======================================
  Files          34       34           
  Lines        7516     7516           
=======================================
  Hits         6394     6394           
  Misses       1122     1122           

Impacted Files	Coverage Δ
datashader/__init__.py	80.00% <100.00%> (ø)
datashader/bundling.py	98.62% <100.00%> (ø)
datashader/colors.py	97.43% <100.00%> (ø)
datashader/compiler.py	96.00% <100.00%> (ø)
datashader/composite.py	96.19% <100.00%> (ø)
datashader/core.py	88.11% <100.00%> (ø)
datashader/data_libraries/cudf.py	57.14% <100.00%> (ø)
datashader/data_libraries/dask.py	94.49% <100.00%> (ø)
datashader/data_libraries/dask_cudf.py	57.14% <100.00%> (ø)
datashader/data_libraries/pandas.py	100.00% <100.00%> (ø)
... and 16 more

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