Fix test that used sympy antipattern

calling a symbol which resulted in a Function was introduced
as a workaround for sympy internals and these were fixed in sympy 1.3
and calling a symbol was removed.

test/test_pymbolic.py

@@ -95,7 +95,7 @@ def test_sympy_interaction():
     import sympy as sp
 
     x, y = sp.symbols("x y")
-    f = sp.symbols("f")
+    f = sp.Function("f")
 
     s1_expr = 1/f(x/sp.sqrt(x**2+y**2)).diff(x, 5)