Fix python version to stabilise launch of mlflow service in CI

[ablaom]

Attempt to address #27

[ablaom]

@pebeto I tried to launch the service as in MLFlowClient CI, and now the service appears to launch okay, but the test to save an artifact fails to find an artifact.

Can you please help sort out this issue?

[pebeto]

@ablaom the MLFlowClient CI configuration for mlflow just test the existence of an artifact, it doesn't retrieve it. To get that working, we need to start mlflow in the same instance running MLJFlow.

I'm going to review it.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (f7423ca) 97.05% compared to head (4005b79) 97.77%.
Report is 7 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #28      +/-   ##
==========================================
+ Coverage   97.05%   97.77%   +0.71%     
==========================================
  Files           4        4              
  Lines          34       45      +11     
==========================================
+ Hits           33       44      +11     
  Misses          1        1              

Files	Coverage Δ
src/MLJFlow.jl	100.00% <ø> (ø)
src/base.jl	100.00% <ø> (ø)
src/service.jl	100.00% <100.00%> (ø)

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