* Add dependabot config
This commit adds a dependabot config to automatically open pull requests
with version bumps for dependencies. This will configure dependabot to
check daily if any pyo3 dependency versions are out of date and if they
are it will open a PR for each dependency. You can find more details on
the dependabot configuration and usage in in the github
documentation. [1]
[1] https://docs.github.com/en/github/administering-a-repository/enabling-and-disabling-version-updates
* Switch to weekly update interval
