Use python3.9 in CI (Project-MONAI#599)

Fixes Project-MONAI#598 .

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

### Status
**Ready/Work in progress/Hold**

### Please ensure all the checkboxes:
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [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"`).

---------

Signed-off-by: Yiheng Wang <vennw@nvidia.com>

.github/workflows/code-format-check.yml

@@ -17,10 +17,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: cache weekly timestamp
       id: pip-cache
       run: |

ci/run_premerge_gpu.sh

@@ -33,7 +33,7 @@ fi
 init_pipenv() {
     echo "initializing pip environment: $1"
     pipenv install update pip wheel
-    pipenv install --python=3.8 -r $1
+    pipenv install --python=3.9 -r $1
     export PYTHONPATH=$PWD
 }
 

ci/run_premerge_multi_gpu.sh

@@ -33,7 +33,7 @@ fi
 init_pipenv() {
     echo "initializing pip environment: $1"
     pipenv install update pip wheel
-    pipenv install --python=3.8 -r $1
+    pipenv install --python=3.9 -r $1
     export PYTHONPATH=$PWD
 }