update global dependabot config to include python/pip/requirements.txt

[jeffsays]

we added a requirements.txt to clark kent in https://github.com/CumulusDS/clark-kent/pull/611 to lock the version of weasyPrint on v66 due to issues with v67

we should keep an eye on versions here with our depenabot configuration

[jeffsays]

https://docs.github.com/en/code-security/reference/supply-chain-security/supported-ecosystems-and-repositories

https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference