Fix gulp install python libs commands with python

[kimadeline]

Fix installPtvsdWheels and installOldPtvsd commands when run on a machine that doesn't have an alias for python3.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Appropriate comments and documentation strings in the code
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)
• The wiki is updated with any design decisions/details.

[codecov-io]

Codecov Report

Merging #8282 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #8282      +/-   ##
=========================================
+ Coverage   59.01%   59.1%   +0.08%     
=========================================
  Files         505     505              
  Lines       23124   23154      +30     
  Branches     3741    3754      +13     
=========================================
+ Hits        13647   13685      +38     
+ Misses       8596    8579      -17     
- Partials      881     890       +9

Impacted Files	Coverage Δ
src/client/datascience/jupyter/jupyterImporter.ts	13.2% <0%> (-5.22%)	⬇️
src/client/datascience/webViewHost.ts	48.18% <0%> (+0.9%)	⬆️
.../datascience/interactive-common/interactiveBase.ts	24.33% <0%> (+4.23%)	⬆️
...ient/datascience/interactive-ipynb/nativeEditor.ts	58.19% <0%> (+4.34%)	⬆️

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