Behaviour before this PR:
If phi is a Drinfeld module, the method phi.ore_polring() used to return an Ore polynomial ring with coefficients in RingExtension.

This causes a lot of headache because currently an instance of RingExtension does not always belong to the appropriate category and, consequently, does not implement all relevant methods and/or is not recognized as what it actually is (e.g. a finite field).

In this PR, we propose a simple fix: we let phi.ore_polring() return an Ore polynomial ring over the underlying regular field. This only changes the way things are printed, but does not alter the user interface.

We will maybe come back later to the original implementation when RingExtension will be fixed (which does not seem to be straightforward).

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.