Make the sphinx extension tests more robust to manually building the docs in tinypages

The tests now don't copy any of the build files, meaning the tests should pass
even if tinypages has stale build state from a manual build.

Author: asmeurer

lib/matplotlib/tests/data/tinypages/.gitignore

@@ -1 +1,3 @@
 _build/
+doctrees/
+plot_directive/

lib/matplotlib/tests/test_sphinxext.py

@@ -16,6 +16,17 @@
 tinypages = Path(__file__).parent / 'data/tinypages'
 
 
+def _ignore_build_artifacts(dir, files):
+    """
+    Ignore function for shutil.copytree to exclude build artifacts.
+
+    This prevents stale build artifacts from the source tinypages directory
+    from being copied to test directories, which could cause inconsistent
+    test results due to cached or outdated files.
+    """
+    return {'_build', 'doctrees', 'plot_directive'} & set(files)
+
+
 def build_sphinx_html(source_dir, doctree_dir, html_dir, extra_args=None):
     # Build the pages with warnings turned into errors
     extra_args = [] if extra_args is None else extra_args
@@ -40,7 +51,8 @@ def build_sphinx_html(source_dir, doctree_dir, html_dir, extra_args=None):
 
 
 def test_tinypages(tmp_path):
-    shutil.copytree(tinypages, tmp_path, dirs_exist_ok=True)
+    shutil.copytree(tinypages, tmp_path, dirs_exist_ok=True,
+                    ignore=_ignore_build_artifacts)
     html_dir = tmp_path / '_build' / 'html'
     img_dir = html_dir / '_images'
     doctree_dir = tmp_path / 'doctrees'
@@ -204,11 +216,13 @@ def test_plot_html_show_source_link_custom_basename(tmp_path):
 
 
 def test_srcset_version(tmp_path):
+    shutil.copytree(tinypages, tmp_path, dirs_exist_ok=True,
+                    ignore=_ignore_build_artifacts)
     html_dir = tmp_path / '_build' / 'html'
     img_dir = html_dir / '_images'
     doctree_dir = tmp_path / 'doctrees'
 
-    build_sphinx_html(tinypages, doctree_dir, html_dir,
+    build_sphinx_html(tmp_path, doctree_dir, html_dir,
                       extra_args=['-D', 'plot_srcset=2x'])
 
     def plot_file(num, suff=''):