Refactor Expr to standard Python class

Changed Expr from a Cython cdef class to a standard Python class for improved compatibility and maintainability. Removed cdef public dict children, as attribute is now managed in Python.

Author: Zeroto521

src/pyscipopt/expr.pxi

@@ -59,11 +59,9 @@ cdef float _evaluate(dict children, SCIP* scip, SCIP_SOL* sol):
     return sum([i._evaluate(scip, sol) * j for i, j in children.items() if j != 0])
 
 
-cdef class Expr:
+class Expr:
     """Base class for mathematical expressions."""
 
-    cdef public dict children
-
     def __init__(self, children: Optional[dict] = None):
         self.children = children or {}