gh-39275: Specify use_sources=True in cython compilation
    
In my testing, this allows cython compilation of code with `from
sage.[...] cimport [...]` to work when built with meson. Ability to
compile cython code when use legacy build system is unaffected.

Actually without this change cython compilation works fine on conda &
meson CI anyway (probably because of some coincidence, I didn't inspect)

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### ⌛ Dependencies

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URL: #39275
Reported by: user202729
Reviewer(s): Tobias Diez

Author: Release Manager

src/sage/misc/cython.py

@@ -60,7 +60,7 @@ def _standard_libs_libdirs_incdirs_aliases():
     if SAGE_LOCAL:
         standard_libdirs.append(os.path.join(SAGE_LOCAL, "lib"))
     standard_libdirs.extend(aliases["CBLAS_LIBDIR"] + aliases["NTL_LIBDIR"])
-    standard_incdirs = sage_include_directories() + aliases["CBLAS_INCDIR"] + aliases["NTL_INCDIR"]
+    standard_incdirs = sage_include_directories(use_sources=True) + aliases["CBLAS_INCDIR"] + aliases["NTL_INCDIR"]
     return standard_libs, standard_libdirs, standard_incdirs, aliases
 
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