Implement automatic package updates

[scaramallion]

• Figure out how to automate opening a PR
• Can the automatic PR also be automatically merged?
• Figure out how to trigger the PR workflow on pydicom merge workflow completion
• Set package version automatically
• (Later) Trigger deploy workflow on pydicom deploy workflow success

[scaramallion]

OK, some notes:

1. Enabled organisation level fine-grained personal access tokens.
2. Create a new personal access token: My Profile -> Settings -> Developer Settings -> Personal Access Tokens -> Fine-grained tokens -> Generate new token
3. Name it something obvious
4. Expiration: something useful, 90 days for now but will need to be updated as needed
5. Resource owner: change to the pydicom organisation
6. Only selected repositories: pydicom/types-pydicom
7. Repository permissions: Actions (RW), Contents (RW), Metadata (R), Pull Requests (RW)
8. Generate token, copy the value
9. In pydicom/types-pydicom, go to settings and add a new secret, the value should be the token value from the previous step
10. In the receiver.yml workflow, the secrets token name should match the one in the types-pydicom settings
11. In pydicom, add/update the secret used by the trigger workflow using the same token value.