Remove `sage/ext` from `sage_include_directories`

[mkoeppe]

The special casing of this directory (in sage.env.sage_include_directories) seems outdated.

We remove it and change the single include directive that relied on it.

CC: @tobiasdiez @kiwifb @jhpalmieri

Component: refactoring

Author: Tobias Diez, Matthias Koeppe

Branch/Commit: 6ccce6b

Reviewer: Matthias Koeppe, Tobias Diez

Issue created by migration from https://trac.sagemath.org/ticket/30672

[mkoeppe]

Author: Tobias Diez, Matthias Koeppe

[mkoeppe]

Branch: u/mkoeppe/remove__sage_ext__from__sage_include_directories_

[mkoeppe]

New commits:

52e2107	sage_include_directories: Remove sage/ext
6ccce6b	Fix missing ccobject

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1,5 +1,5 @@
 The special casing of this directory (in `sage.env.sage_include_directories`) seems outdated.
 
-We remove it and change the (few) includes that relied on it.
+We remove it and change the single include directive that relied on it.
 
 

[mkoeppe]

Commit: 6ccce6b

[tobiasdiez]

comment:4

Thanks for extracting this from #30371. The changes look good to me.

[mkoeppe]

Reviewer: Matthias Koeppe, Tobias Diez

[vbraun]

Changed branch from u/mkoeppe/remove__sage_ext__from__sage_include_directories_ to 6ccce6b