MAINT: use st_mtime for tracking modifications

[docs only]

doc/convert_notebooks.py

@@ -14,13 +14,14 @@ def call_jupytext(md_files, _contents_path, _contents_cache_path):
         return True
 
     is_dirty = False
+
     for md_file in md_files:
         basename = os.path.splitext(os.path.basename(md_file))[0]
         output_path = os.path.join(_contents_path, f"{basename}.ipynb")
         cached_output_path = os.path.join(_contents_cache_path, f"{basename}.ipynb")
-        cmd_execution_time = os.path.getctime(cached_output_path)
-        md_file_modification_time = os.path.getmtime(md_file)
-        if cmd_execution_time <= md_file_modification_time:
+        cmd_execution_time = os.stat(cached_output_path).st_mtime
+        md_file_modification_time = os.stat(md_file).st_mtime
+        if cmd_execution_time < md_file_modification_time:
             nb = jupytext.read(md_file)
             jupytext.write(nb, output_path, fmt="ipynb", version=4)
             is_dirty = True
@@ -35,6 +36,7 @@ def call_jupytext(md_files, _contents_path, _contents_cache_path):
 if __name__ == '__main__':
     _contents_cache_path = os.path.join("build", "_contents")
     _contents_path = os.path.join("source", "_contents")
+
     os.makedirs(os.path.expanduser(_contents_path), exist_ok=True)
 
     md_files = glob.glob("source/tutorial/stats/*.md")