update name for Python 3 image

[dgrove-oss]

We changed the image name we use to publish on dockerhub
from actionloop-python-v3.7 to action-python-v3.7 and
forgot to update runtime.json to match.

[codecov-commenter]

Codecov Report

Merging #5148 (4b70293) into master (91e3f28) will increase coverage by 34.95%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5148       +/-   ##
===========================================
+ Coverage   39.88%   74.83%   +34.95%     
===========================================
  Files         231      231               
  Lines       12803    12803               
  Branches      501      501               
===========================================
+ Hits         5106     9581     +4475     
+ Misses       7697     3222     -4475     

Impacted Files	Coverage Δ
...ntainerpool/v2/FunctionPullingContainerProxy.scala	78.12% <0.00%> (+0.34%)	⬆️
...pache/openwhisk/core/invoker/InvokerReactive.scala	58.62% <0.00%> (+0.86%)	⬆️
...la/org/apache/openwhisk/common/TransactionId.scala	94.38% <0.00%> (+2.24%)	⬆️
.../org/apache/openwhisk/http/PoolingRestClient.scala	90.90% <0.00%> (+3.03%)	⬆️
...re/database/MultipleReadersSingleWriterCache.scala	98.07% <0.00%> (+3.84%)	⬆️
...in/scala/org/apache/openwhisk/common/Logging.scala	77.07% <0.00%> (+3.90%)	⬆️
...a/org/apache/openwhisk/core/database/Batcher.scala	95.00% <0.00%> (+5.00%)	⬆️
...penwhisk/core/containerpool/ContainerFactory.scala	88.88% <0.00%> (+5.55%)	⬆️
...la/org/apache/openwhisk/core/entity/LogLimit.scala	94.11% <0.00%> (+5.88%)	⬆️
...pache/openwhisk/core/entity/ConcurrencyLimit.scala	94.11% <0.00%> (+5.88%)	⬆️
... and 141 more

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