sagemathgh-40001: Remove giac as dependency, and add sagemath_giac as optional dependency
    
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Not sure if this change is completely right. It seems to me that there
is still a giac expect interface in use, so perhaps giac should now be a
runtime dependency instead of build dependency of sagelib? Or is that
interface obsolete and should/can be removed?

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URL: sagemath#40001
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager

pyproject.toml

@@ -75,6 +75,9 @@ requires-python = ">=3.11, <3.14"
 R = [
     'rpy2 >=3.3',
 ]
+giac = [
+    'sagemath_giac',
+]
 
 [project.readme]
 file = "README.md"
@@ -120,7 +123,6 @@ host-requires = [
   "pkg:generic/libgd",
   "pkg:generic/gap",
   "pkg:generic/gfan",
-  "pkg:generic/giac",
   "pkg:generic/givaro",
   "pkg:generic/glpk",
   "pkg:generic/gmp",