fix: symlink python3, not python, to python3.7

[tk-woven]

Description

This addresses #105 . For Ubuntu 18, default python is 2.7 and python3 is 3.x, so we need to symlink python3 and not python.

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This change is 

[chrisochoatri]

Reviewed 1 of 1 files at r1, all commit messages.
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[kuanleetri]

Interesting, why we need to specifically assign 3? I thought by default is 3.7?

Reviewed all commit messages.
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[kuanleetri]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @tk-woven)

[tk-woven]

Thanks for the review! The Docker image uses Ubuntu 18. Python3.6 is the default python3 for Ubuntu18. My understanding from the Dockerfile is that we want to use Python3.7, so we need to set that up manually. Otherwise python3 will point to 3.6.

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @tk-woven)