Merge pull request matplotlib#20751 from meeseeksmachine/auto-backport-of-pr-20748-on-v3.4.x

Backport PR matplotlib#20748 on branch v3.4.x (Ensure _static directory exists before copying CSS.)

Author: QuLogic

lib/matplotlib/sphinxext/plot_directive.py

@@ -258,6 +258,7 @@ def _copy_css_file(app, exc):
     if exc is None and app.builder.format == 'html':
         src = cbook._get_data_path('plot_directive/plot_directive.css')
         dst = app.outdir / Path('_static')
+        dst.mkdir(exist_ok=True)
         shutil.copy(src, dst)