Use SAGE_LIB instead of of a path derived from SAGE_LOCAL in sage/doctest/sources.py

[kiwifb]

In src/sage/doctest/sources.py the following code

sp = os.path.join(SAGE_LOCAL, 'lib', 'python', 'site-packages')

can easily be replaced by SAGE_LIB which is the intended location in any case.

Note that this is also the only call to SAGE_LOCAL in this file.

Component: doctest framework

Author: François Bissey

Branch/Commit: 6ccfd3a

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/27191

[kiwifb]

New commits:

6ccfd3a

use SAGE_LIB instead of SAGE_LOCAL/lib/python/site-packages.

[kiwifb]

Branch: u/fbissey/sage_lib

[kiwifb]

Author: François Bissey

[kiwifb]

Commit: 6ccfd3a

[kiwifb]

Description changed:

--- 
+++ 
@@ -5,3 +5,4 @@
 ```
 can easily be replaced by `SAGE_LIB` which is the intended location in any case.
 
+Note that this is also the only call to `SAGE_LOCAL` in this file.

[jdemeyer]

Reviewer: Jeroen Demeyer

[vbraun]

Changed branch from u/fbissey/sage_lib to 6ccfd3a