gh-39621: Fix documentation diff for cython()
    
As discussed in
#38946 (comment) ,
this hides an irrelevant entry in documentation diff.

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URL: #39621
Reported by: user202729
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/misc/cython.py

@@ -88,7 +88,7 @@ def _webbrowser_open_file(path):
 
 def cython(filename, verbose=0, compile_message=False,
            use_cache=False, create_local_c_file=False, annotate=True, view_annotate=False,
-           view_annotate_callback=_webbrowser_open_file, sage_namespace=True, create_local_so_file=False):
+           view_annotate_callback=None, sage_namespace=True, create_local_so_file=False):
     r"""
     Compile a Cython file. This converts a Cython file to a C (or C++ file),
     and then compiles that. The .c file and the .so file are
@@ -124,7 +124,8 @@ def cython(filename, verbose=0, compile_message=False,
     - ``view_annotate_callback`` -- function; a function that takes a string
       being the path to the html file. This can be overridden to change
       what to do with the annotated html file. Have no effect unless
-      ``view_annotate`` is ``True``.
+      ``view_annotate`` is ``True``. By default, the html file is opened in a
+      web browser.
 
     - ``sage_namespace`` -- boolean (default: ``True``); if ``True``, import
       ``sage.all``
@@ -271,6 +272,9 @@ def cython(filename, verbose=0, compile_message=False,
         sage: len(collected_paths)
         1
     """
+    if view_annotate_callback is None:
+        # needed because of https://github.com/sagemath/sage/pull/38946#issuecomment-2656329774
+        view_annotate_callback = _webbrowser_open_file
     if not filename.endswith('pyx'):
         print("Warning: file (={}) should have extension .pyx".format(filename), file=sys.stderr)