Change __call__ to _element_constructor_ in multipolynomial ring

[user202729]

Just a trivial change. It's not recommended to override __call__.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have created tests covering the changes. (probably not needed)
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit aea07fd; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[user202729]

Hm why are tests failing everywhere…? (obviously must have something to do with the coercion system etc.)

---

Apparently MPolynomialRing_base never had _coerce_map_from_ implemented (only the lib singular ring has), and somehow nobody notices something is wrong.