Add snippet for allocation and initialisation behaviour

blog_post/new_init.pyx

+cdef cypclass Base:
+    double value
+
+    __init__(self, int a):
+        self.value = (<double> a ) * 1.2
+
+    __init__(self, double b):
+        self.value = b
+
+    Base __new__(alloc, int a):
+        obj = alloc()
+        return obj
+
+
+cdef cypclass Derived(Base):
+    Derived __new__(alloc, double b):
+        obj = alloc()
+        return obj
+
+cpdef void test():
+    base = Base(5)
+    derived = Derived(5)
+
+    print(base.value, derived.value)
+