.codespellrc: ignore intersphinx inventories (*.inv)

We don't want to spell check these. We have removed the in-repo
intersphinx inventories from devel, but they exist on older branches.
There's no harm in also including the ignores on devel, and it makes
sense to keep the configuration consistent.

.codespellrc

@@ -1,5 +1,5 @@
 [codespell]
-skip = _build,collections,*.po,_static,porting_guide*,style_guide
+skip = _build,collections,*.po,_static,porting_guide*,style_guide,*.inv
 count =
 check-filenames = true
 quiet-level = 3