change the mod method of rationals

[videlec]

The current behavior of mod on Sage rationals is currently useless

sage: (2/3).mod(5)
0
sage: (2/3).mod(6)
0

And very different from what mod_ui does

sage: (2/3).mod_ui(5)
4
sage: (2/3).mod_ui(6)
Traceback (most recent call last):
...
ArithmeticError: The inverse of 3 modulo 6 is not defined.

This ticket stands for:

• changing the behavior of mod on rationals to be consistent to the current mod_ui (and handle the potential overflow in mod_ui)
• deprecate mod_ui in favor of mod

This change will make Sage compatible with PARI/GP Mod and maple mod.

Component: basic arithmetic

Issue created by migration from https://trac.sagemath.org/ticket/21748

[videlec]

Description changed:

--- 
+++ 
@@ -6,7 +6,7 @@
 sage: (2/3).mod(6)
 0
 ```
-And very different from `mod_ui` does
+And very different from what `mod_ui` does
 
 ```
 sage: (2/3).mod_ui(5)

[videlec]

Description changed:

--- 
+++ 
@@ -17,4 +17,8 @@
 ArithmeticError: The inverse of 3 modulo 6 is not defined.
 ```
 
-This ticket stands for changing the behavior of `mod` on rationals to be consistent to `mod_ui`. That would also make it compatible with PARI/GP `Mod` and maple `mod`.
+This ticket stands for:
+- changing the behavior of `mod` on rationals to be consistent to the current `mod_ui` (and handle the potential overflow in `mod_ui`)
+- deprecate `mod_ui` in favor of `mod`
+
+This change will make Sage compatible with PARI/GP `Mod` and maple `mod`.