gh-39776: Safeguard _sage_getargspec_cython
    
Fix #39735

Actually just a workaround, this sweeps the fundamental problem under
the rug.

Without source available:

```
sage: from sage.structure.category_object import CategoryObject
sage: CategoryObject._CategoryObject__gens_dict
Cached version of <method '_CategoryObject__gens_dict' of
'sage.structure.category_object.CategoryObject' objects>
sage: type(CategoryObject._CategoryObject__gens_dict)
<class 'sage.misc.cachefunc.CachedMethodCaller'>
```

With source available (specialized version `CachedMethodCallerNoArgs` is
used, thus more efficient):

```
sage: from sage.structure.category_object import CategoryObject
sage: CategoryObject._CategoryObject__gens_dict
Cached version of <method '_CategoryObject__gens_dict' of
'sage.structure.category_object.CategoryObject' objects>
sage: type(CategoryObject._CategoryObject__gens_dict)
<class 'sage.misc.cachefunc.CachedMethodCallerNoArgs'>
```

The proper fix should be to either somehow make `sage_getargspec` always
return the correct value,
or to somehow make the cache function mechanism know when to use
specialization.

In any case, it can't hurt to make the function raise the correct
exception type.

### 📝 Checklist

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- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #39776
Reported by: user202729
Reviewer(s): Tobias Diez, user202729

Author: Release Manager

src/sage/misc/sageinspect.py

@@ -1140,6 +1140,10 @@ def _sage_getargspec_cython(source):
                     defaults=('a string', {(1, 2, 3): True}),
                     kwonlyargs=[], kwonlydefaults=None, annotations={})
     """
+    assert isinstance(source, str)
+    # the caller ought to ensure this, but if it forgets (e.g. passing None),
+    # we avoid raising AttributeError to avoid confusing error message
+    # and possible further hard-to-debug errors, see :issue:`39735`
     defpos = source.find('def ')
     assert defpos > -1, "The given source does not contain 'def'"
     s = source[defpos:].strip()
@@ -1677,12 +1681,15 @@ def foo(x, a='\')"', b={not (2+1==3):'bar'}): return
         except TypeError:  # arg is not a code object
             # The above "hopefully" was wishful thinking:
             try:
-                return inspect.FullArgSpec(*_sage_getargspec_cython(sage_getsource(obj)))
+                source = sage_getsource(obj)
             except TypeError:  # This happens for Python builtins
-                # The best we can do is to return a generic argspec
-                args = []
-                varargs = 'args'
-                varkw = 'kwds'
+                source = None
+            if source is not None:
+                return inspect.FullArgSpec(*_sage_getargspec_cython(source))
+            # The best we can do is to return a generic argspec
+            args = []
+            varargs = 'args'
+            varkw = 'kwds'
     try:
         defaults = func_obj.__defaults__
     except AttributeError: