MAINT: Support Python 3.9

[larsoner]

base64.decodestring does not exist in Python 3.9 so current master causes an error on import. decodestring has been deprecated since 3.1 so should be safe just to use decodebytes directly (and same with encodebytes).

[codecov-io]

Codecov Report

Merging #983 (c0be6ef) into master (8780225) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #983   +/-   ##
=======================================
  Coverage   49.79%   49.79%           
=======================================
  Files         263      263           
  Lines       23927    23927           
  Branches     3259     3259           
=======================================
  Hits        11914    11914           
  Misses      11236    11236           
  Partials      777      777           

Impacted Files	Coverage Δ
mayavi/tools/remote/ipy_remote.py	0.00% <0.00%> (ø)
mayavi/tools/remote/remote_scene.py	0.00% <0.00%> (ø)

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[rahulporuri]

I think we can simply remove the temporary variables now and just use base64.decodebytes or base64.encodebytes. Can you please make that change?

[larsoner]

@rahulporuri done

[rahulporuri]

LGTM. Thanks @larsoner ! Waiting for CI.

[prabhuramachandran]

@larsoner -- thanks! @rahulporuri -- thanks for merging.