Update python to 3.7.3

What does this MR do?

Updates Python to 3.7.3 to avoid to use EOL-ed Python 3.4.

Counterpart MR: gitlab-org/build/CNG!210 (merged)

Closes #3804 (closed)

Checklist

See Definition of done.

• Change added to CHANGELOG.md. Not applicable for Documentation changes and minor changes.
• [n/a] Documentation created/updated
• [n/a] Tests added
• [n/a] Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com
• Wait for Python 3.7.3 released

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks

References

• Python 3.7 release note: https://docs.python.org/3.7/whatsnew/3.7.html
• Python 3.7.3 schedule: https://www.python.org/dev/peps/pep-0537/