How to group dependancies by file?

[qqii]

For example I have requirements.txt and requirements-dev.txt. I want to group updates to requirements-dev.txt into a single PR, but leave updates to requirements.txt separate.

In essence, I'd like to write something like this:

  - package-ecosystem: "pip"
    directory: "requirements.txt"
  - package-ecosystem: "pip"
    directory: "requirements-dev.txt"
    groups:
      all:
        patterns:
          - "*"

But this fails with the following error:

+---------------------------------------------------------------------------------+
|                                     Errors                                      |
+---------------------------+-----------------------------------------------------+
| Type                      | Details                                             |
+---------------------------+-----------------------------------------------------+
| dependency_file_not_found | {                                                   |
|                           |   "message": "No files found in /requirements.txt", |
|                           |   "file-path": null                                 |
|                           | }                                                   |
+---------------------------+-----------------------------------------------------+

Is there a good way to do this?