update default python3 image name

[dgrove-oss]

We stopped building the old (non-actionloop) python 3 docker image
about six months ago. Update the default image name so we use/test the
actionloop-based python3 runtime.

[codecov-commenter]

Codecov Report

Merging #5104 (c7eda77) into master (cd6fded) will increase coverage by 41.75%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5104       +/-   ##
===========================================
+ Coverage   32.52%   74.27%   +41.75%     
===========================================
  Files         211      211               
  Lines       10435    10435               
  Branches      447      447               
===========================================
+ Hits         3394     7751     +4357     
+ Misses       7041     2684     -4357     

Impacted Files	Coverage Δ
.../openwhisk/core/database/cosmosdb/cache/Main.scala	0.00% <0.00%> (ø)
...a/org/apache/openwhisk/core/controller/Rules.scala	89.33% <0.00%> (+89.33%)	⬆️
...he/openwhisk/core/invoker/InstanceIdAssigner.scala	97.22% <0.00%> (+97.22%)	⬆️
...core/database/cosmosdb/RxObservableImplicits.scala	0.00% <0.00%> (ø)
...la/org/apache/openwhisk/core/invoker/Invoker.scala	69.84% <0.00%> (ø)
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0.00% <0.00%> (ø)
...sk/core/database/cosmosdb/CosmosDBViewMapper.scala	0.00% <0.00%> (ø)
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0.00% <0.00%> (ø)
...rg/apache/openwhisk/core/scheduler/Scheduler.scala	0.00% <0.00%> (ø)
...whisk/core/database/cosmosdb/CosmosDBSupport.scala	0.00% <0.00%> (ø)
... and 82 more

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