Remove the bundled 'echo' package

[astrofrog]

This removes the bundled version of echo in favor of the standalone package. For now, this requires the developer version of echo since I don't yet have permission to upload new releases to PyPI.

[codecov]

Codecov Report

Merging #2125 into master will increase coverage by 0.01%.
The diff coverage is 96.36%.

@@            Coverage Diff             @@
##           master    #2125      +/-   ##
==========================================
+ Coverage   87.99%   88.00%   +0.01%     
==========================================
  Files         245      245              
  Lines       22322    22322              
==========================================
+ Hits        19642    19645       +3     
+ Misses       2680     2677       -3     

Impacted Files	Coverage Δ
glue/_deps.py	66.38% <ø> (ø)
glue/plugins/dendro_viewer/layer_style_editor.py	0.00% <0.00%> (ø)
glue/utils/qt/helpers.py	53.93% <ø> (ø)
glue/viewers/image/qt/slice_widget.py	72.34% <0.00%> (ø)
glue/app/qt/save_data.py	92.23% <100.00%> (ø)
glue/core/callback_property.py	100.00% <100.00%> (ø)
glue/core/data_combo_helper.py	91.53% <100.00%> (ø)
glue/core/layer_artist.py	80.00% <100.00%> (ø)
glue/core/state_objects.py	90.87% <100.00%> (ø)
glue/core/visual.py	84.44% <100.00%> (ø)
... and 37 more

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