sagemathgh-37395: `build/pkgs/mathics`: Reduce to a pip package
    
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Our Mathics package is outdated (and also the interface shows test
failures in CI - e.g. in https://github.com/sagemath/sage/actions/runs/7
894201915/job/21638423818#step:14:3279)

Trying to update Mathics reveals more dependencies, include scikit-image
and llvm-lite.

Here we reduce it to a "pip" package to simplify maintenance of the
package on our side.

The latest released version 6.0.4 fails to install in Sage because of
rigid version constraints.

We are using the current HEAD version (7.0.0dev), which includes
- Mathics3/mathics-core#1014
- Mathics3/mathics-core#1034

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URL: sagemath#37395
Reported by: Matthias Köppe
Reviewer(s): Matthias Köppe, Sebastian Oehms