sagemathgh-36590: Allow to import `sage.cpython` module multiple times
    
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I don't really know why but pytest is sometimes importing the same
module multiple times (I think once for the test collection and once for
the execution). Importing cpython twice however fails since the second
time `_ExtensionFileLoader.get_source` is already deleted.

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URL: sagemath#36590
Reported by: Tobias Diez
Reviewer(s): Matthias Köppe

src/sage/cpython/__init__.py

@@ -12,7 +12,8 @@
 # Monkey-patch ExtensionFileLoader to allow IPython to find the sources
 # of Cython files. See https://github.com/sagemath/sage/issues/24681
 from importlib.machinery import ExtensionFileLoader as _ExtensionFileLoader
-del _ExtensionFileLoader.get_source
+if hasattr(_ExtensionFileLoader, 'get_source'):
+    del _ExtensionFileLoader.get_source
 del _ExtensionFileLoader
 
 # Work around a Cygwin-specific bug caused by sqlite3; see