Read secrets from another file

[fioddor]

Several data source configurations use secrets like the field api-token.

These ought to be collected from another file (or files), so the configuration can be safely published. The prominent use case is sharing it on public forums in the context of support conversations.

setup.cfg:

credentials=path/to/file
...

[github:issue]
...

[gitlab:issue]
...

[my_self_hosted_gitlab:issue]
...

path/to/file:

[github:issue]
api-token=a_long_and_secure_api_token

[gitlab:issue]
api-token=another_api_token

[my_self_hosted_gitlab:issue]
api-token=yet_another_api_token