Trac #32185: special case constant symbolic relations.

The mixed_order() code in sage/symbolic/comparison.pyx uses a bunch
of heuristics to figure out how to compare two things. To compare
(say) x and y, it creates the expression (x - y), and then looks at
what type of thing (x - y) is. In most cases, this seems to work;
however, when x is a symbolic constant and y is zero, the expression
(x - y) reduces to just x, a symbolic constant. The mixed_order()
code is not expecting this, and it fails to compare x.pyobject()
to the integer zero.

To work around this problem, this commit adds an additional check for
whether or not (x - y) is a symbolic constant, and falls through to a
numerical comparison in that case.

src/sage/symbolic/comparison.pyx

@@ -258,6 +258,16 @@ cpdef int mixed_order(lhs, rhs) except -2:
 
         sage: mixed_order(SR(oo), sqrt(2))
         1
+
+    Ensure that :trac:`32185` is fixed::
+
+        sage: mixed_order(pi, 0)
+        1
+        sage: mixed_order(golden_ratio, 0)
+        1
+        sage: mixed_order(log2, 0)
+        1
+
     """
     if lhs is rhs:
         return 0
@@ -345,7 +355,7 @@ class _mixed_key(object):
             return pynac_result == relational_true
 
         det_ex = self.ex - other.ex
-        if not has_symbol_or_function((<Expression>rel)._gobj):
+        if not has_symbol_or_function((<Expression>rel)._gobj) and not det_ex.is_constant():
             while hasattr(det_ex, 'pyobject') and isinstance(det_ex, Expression):
                 try:
                     det_ex = det_ex.pyobject()