RealSet: deprecate using a, b to construct the open interval (a, b) in union and intersection methods

[yuan-zhou]

Before:

sage: s1 = RealSet(0,2)
sage: s1.union(1,3)
(0, 3)

After, s1.union(1,3) is no longer allowed.
Must be written as
sage: s1.union((1,3)) or s1.union(RealSet(1,3)) or s1.union(RealSet.open(1,3)), etc.

See #32181 comment:58

Depends on #32181

CC: @mkoeppe

Component: basic arithmetic

Issue created by migration from https://trac.sagemath.org/ticket/34280

[yuan-zhou]

Description changed:

--- 
+++ 
@@ -8,3 +8,5 @@
 After, `s1.union(1,3)` is no longer allowed. 
 Must be written as
 `sage: s1.union((1,3))` or `s1.union(RealSet(1,3))` or `s1.union(RealSet.open(1,3))`, etc.
+
+See [#32181 comment:58](https://github.com/sagemath/sage/issues/32181#comment:58)