Update Python versions

[pabloge]

Update Python versions to those supported by GitHub Actions. Currently, Python 2.7 is causing this error in the Python package build job (e.g. in #25):

Warning: The support for python 2.7 will be removed on June 19. Related issue: https://github.com/actions/setup-python/issues/672
Version 2.7 was not found in the local cache
Error: Version 2.7 with arch x[6](https://github.com/toumorokoshi/deepmerge/actions/runs/7224927610/job/19711710348?pr=25#step:4:7)4 not found
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

[toumorokoshi]

thanks!