gh-35275: Drinfeld modules: Make some imports lazy
    
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### 📚 Description

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We replace some `import`s by `lazy_imports` in
`sage.categories.drinfeld_modules` and make the import of
`DrinfeldModule` into the global namespace lazy. We also move the
module-level import of `PolynomialBaseringInjection` in
`sage.rings.polynomial.polynomial_ring` into a method.
<!-- Why is this change required? What problem does it solve? -->

`git grep '^from sage.rings' src/sage/categories` reveals that the
recently added `.drinfeld_modules` is the only module that imports
nontrivial things from `sage.rings`. This is a new obstacle to
modularization (it caused an error due to import cycles in #35095.)

The purpose of reducing the module-level import to a method-level import
is explained in https://doc.sagemath.org/html/en/developer/packaging_sag
e_library.html#module-level-runtime-dependencies

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example "Closes #1337" -->

### 📝 Checklist

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### ⌛ Dependencies
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URL: #35275
Reported by: Matthias Köppe
Reviewer(s): Antoine Leudière, Matthias Köppe

Author: Release Manager

src/sage/categories/drinfeld_modules.py

@@ -23,10 +23,12 @@
 from sage.categories.homsets import Homsets
 from sage.misc.functional import log
 from sage.misc.latex import latex
+from sage.misc.lazy_import import lazy_import
 from sage.rings.integer import Integer
-from sage.rings.polynomial.ore_polynomial_ring import OrePolynomialRing
-from sage.rings.polynomial.polynomial_ring import PolynomialRing_general
-from sage.rings.ring_extension import RingExtension_generic
+
+lazy_import('sage.rings.polynomial.ore_polynomial_ring', 'OrePolynomialRing')
+lazy_import('sage.rings.polynomial.polynomial_ring', 'PolynomialRing_general')
+lazy_import('sage.rings.ring_extension', 'RingExtension_generic')
 
 
 class DrinfeldModules(Category_over_base_ring):

src/sage/rings/function_field/all.py

@@ -1,2 +1,5 @@
 from .constructor import FunctionField
-from .drinfeld_modules.drinfeld_module import DrinfeldModule
+
+from sage.misc.lazy_import import lazy_import
+
+lazy_import("sage.rings.function_field.drinfeld_modules.drinfeld_module", "DrinfeldModule")

src/sage/rings/polynomial/polynomial_ring.py

@@ -166,8 +166,6 @@
 import sage.rings.abc
 from sage.rings.fraction_field_element import FractionFieldElement
 from sage.rings.finite_rings.element_base import FiniteRingElement
-
-from .polynomial_element import PolynomialBaseringInjection
 from .polynomial_real_mpfr_dense import PolynomialRealDense
 from .polynomial_integer_dense_flint import Polynomial_integer_dense_flint
 from sage.rings.polynomial.polynomial_singular_interface import PolynomialRing_singular_repr
@@ -679,6 +677,8 @@ def _coerce_map_from_base_ring(self):
                       To:   Univariate Polynomial Ring in x over Rational Field
             sage: R.coerce_map_from(GF(7))
         """
+        from .polynomial_element import PolynomialBaseringInjection
+
         return PolynomialBaseringInjection(self.base_ring(), self)
 
     def _coerce_map_from_(self, P):