Upgrade python to 3.9.

[0mdc]

Motivation and Context

As Python 3.7 is nearing end-of-life, we are upgrading Habitat's minimum Python version to 3.9. Some of our dependencies are deprecating Python 3.7, like PyTorch and NumPy. We will also benefit from performance improvements and overall modernization.

Habitat-lab changeset:

• Moving to python 3.9 habitat-lab#1198

How Has This Been Tested

Tested locally and on CI.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[0mdc]

This somehow makes conda build work on the osx image. We're not exactly sure why.

It adds ~13 minutes to the build stage, but we can live with it for now.
Fortunately, this hack won't be necessary when we move from setup.py install to pip install ..

Thanks @rpartsey for reproducing the issue and finding the workaround.

[aclegg3]

LGTM. Thanks!