Use python3.9 in CI

[yiheng-wang-nv]

Fixes #598 .

Description

A few sentences describing the changes proposed in this pull request.

Status

Ready/Work in progress/Hold

Please ensure all the checkboxes:

• [x] Codeformat tests passed locally by running ./runtests.sh --codeformat.
• [ ] In-line docstrings updated.
• [ ] Update version and changelog in metadata.json if changing an existing bundle.
• [ ] Please ensure the naming rules in config files meet our requirements (please refer to: CONTRIBUTING.md).
• [ ] Ensure versions of packages such as monai, pytorch and numpy are correct in metadata.json.
• [ ] Descriptions should be consistent with the content, such as eval_metrics of the provided weights and TorchScript modules.
• [ ] Files larger than 25MB are excluded and replaced by providing download links in large_file.yml.
• [ ] Avoid using path that contains personal information within config files (such as use /home/your_name/ for "bundle_root").

[yiheng-wang-nv]

/build

[yiheng-wang-nv]

blossom tests pass and uses python3.9, I will revert the bundle change and then this PR can merge. cc @KumoLiu

[yiheng-wang-nv]

/build