Handle Pelican >= 4.0 slugs in feeds
Handle Pelican >= 4.0 slugs in feeds

Since Pelican 4.0, `CATEGORY_FEED_ATOM` and `TAG_FEED_ATOM` are expected
to use `{slug}`, not `%s`. Seems to have been introduced in
getpelican/pelican#2383.

This would cause the following error:

```
CRITICAL: TypeError: not all arguments converted during string formatting
```

This change allows for backwards and forward compatibility.

Cherry picked from commit
getpelican/pelican-themes@be0a4105c4e748b9abe91893544a5362d8a23772.

StevenMaude/pelican-bootstrap3-sm,StevenMaude/pelican-bootstrap3-sm