gh-37179: `sage.categories.schemes`: Fix modularization regression
    
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A module-level import is moved back inside methods so that the
**sagemath-categories** distribution works again.
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URL: #37179
Reported by: Matthias Köppe
Reviewer(s): Lorenz Panny

Author: Release Manager

src/sage/categories/schemes.py

@@ -19,8 +19,6 @@
 from sage.categories.fields import Fields
 from sage.categories.homsets import HomsetsCategory
 
-from sage.schemes.generic.scheme import is_Scheme, is_AffineScheme
-
 
 class Schemes(Category):
     """
@@ -75,6 +73,7 @@ def __classcall_private__(cls, X=None):
             Category of schemes over Integer Ring
         """
         if X is not None:
+            from sage.schemes.generic.scheme import is_Scheme
             if not is_Scheme(X):
                 X = Schemes()(X)
             return Schemes_over_base(X)
@@ -137,6 +136,7 @@ def _call_(self, x):
                       To:   Rational Field
 
         """
+        from sage.schemes.generic.scheme import is_Scheme
         if is_Scheme(x):
             return x
         from sage.schemes.generic.morphism import is_SchemeMorphism
@@ -229,6 +229,7 @@ def __init__(self, base):
             sage: AbelianVarieties(Spec(QQ))
             Category of abelian varieties over Rational Field
         """
+        from sage.schemes.generic.scheme import is_AffineScheme
         if is_AffineScheme(base):
             base = base.coordinate_ring()
         if base not in Fields():