Use Python 3.9 in all CI/CD workflows (#196)

# Description
Use Python 3.9 throughout all CI/CD workflows.

Fixes #193 

## Type of change
- [ ] Bug fix and code cleanup
- [ ] New feature
- [ ] Documentation update
- [x] Testing


## Checklist for the reviewer
This checklist should be used as a help for the reviewer.

- [ ] Is the change limited to one issue?
- [ ] Does this PR close the issue?
- [ ] Is the code easy to read and understand?
- [ ] Do all new feature have an accompanying new test?
- [ ] Has the documentation been updated as necessary?
- [ ] Is the code properly tested?

.github/workflows/ci_cd_updated_main.yml

@@ -18,7 +18,7 @@ jobs:
       update_docs: true
       update_python_api_ref: true
       update_docs_landing_page: true
-      python_version: "3.7"
+      python_version: "3.9"
       doc_extras: "[docs]"
     secrets:
       PAT: ${{ secrets.RELEASE_PAT }}

.github/workflows/ci_check_dependencies.yml

@@ -15,7 +15,7 @@ jobs:
       git_username: "TEAM 4.0[bot]"
       git_email: "Team4.0@SINTEF.no"
       permanent_dependencies_branch: "ci/dependency-updates"
-      python_version: "3.7"
+      python_version: "3.9"
       install_extras: "[dev]"
       pr_labels: "CI/CD,skip_changelog"
     secrets:

.github/workflows/ci_update_dependencies.yml

@@ -19,7 +19,7 @@ jobs:
       pr_labels: "CI/CD,skip_changelog"
       extra_to_dos: "- [ ] Make sure that the PR is **squash** merged, with a sensible commit message."
       update_pre-commit: true
-      python_version: "3.7"
+      python_version: "3.9"
       install_extras: "[pre-commit]"
       skip_pre-commit_hooks: "pylint,pylint-tests"
     secrets: