Add general requirements file

[david-cortes-intel]

Description

Adds a minimal requirements file at the top level. There is currently a file with requirements for testing all of the libraries, but it's hard to find (since it's not at the top level) and the other file contains dependencies that aren't needed for cases like comparing one pull request in oneDAL vs. another.

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Checklist:

Completeness and readability

• I have updated the documentation to reflect the changes or created a separate PR with updates and provided its number in the description, if necessary.
• Git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details).
• I have resolved any merge conflicts that might occur with the base branch.

Testing

• I have run it locally and tested the changes extensively.