Test of sizeof proxy object

[madsbk]

Adding test of sizeof of ProxyObject motivated by rapidsai/cudf#9544, which change the __sizeof__ semantic of cudf.
As far as I can see, ProxyObject should continue working after rapidsai/cudf#9544 has been merged.

[codecov-commenter]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.12     #774   +/-   ##
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  Coverage                ?   89.33%           
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  Files                   ?       15           
  Lines                   ?     1969           
  Branches                ?        0           
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  Hits                    ?     1759           
  Misses                  ?      210           
  Partials                ?        0           

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[quasiben]

@gpucibot merge