Ensure arguments to `PIL.Image.resize()` are integers (#11288)

Update ``copy_image_files_pil`` so that the computation of ``nh`` is
always an integer, as otherwise some calls to ``PIL.Image.resize()``
fail as floats are not allowed

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

sphinx/builders/_epub_base.py

@@ -424,7 +424,7 @@ class EpubBuilder(StandaloneHTMLBuilder):
                 (width, height) = img.size
                 nw = self.config.epub_max_image_width
                 if width > nw:
-                    nh = (height * nw) / width
+                    nh = round((height * nw) / width)
                     img = img.resize((nw, nh), Image.BICUBIC)
             try:
                 img.save(path.join(self.outdir, self.imagedir, dest))