Some URLs containing a readable slug and a trailing `/` character cause unhandled internal errors when visited

### Problem

Some URLs, when visited, cause the site to serve an unstyled "Internal Server Error" page. This is happening for author, edition, and work URLs that have a trailing `/` character following a readable slug that contains non-latin characters.

#### Evidence / Screenshot

#### Relevant URL(s)

Working URL:
https://openlibrary.org/books/OL4210801M/Das_schwerho%CC%88rige_Kind

Broken URL:
https://openlibrary.org/books/OL4210801M/Das_schwerho%CC%88rige_Kind/

### Reproducing the bug

1. Go to https://openlibrary.org/books/OL4210801M/Das_schwerho%CC%88rige_Kind/

* Expected behavior: You are looking at a book page
* Actual behavior: You are looking at an error message

### Context

- Browser (Chrome, Safari, Firefox, etc): Any
- OS (Windows, Mac, etc): Any
- Logged in (Y/N): N
- Environment (prod, dev, local): prod

Related to this Sentry [issue](https://sentry.archive.org/organizations/ia-ux/issues/42549/?query=is%3Aunresolved&referrer=issue-stream)

### Notes from this Issue's Lead

#### Proposal & constraints

We suspect that this error may have something to do with how our ReadableUrlProcessor is encoding the readable slug and redirecting.

A [similar issue](https://github.com/internetarchive/openlibrary/issues/4311) was solved by creating a [`safe_seeother`](https://github.com/internetarchive/openlibrary/blob/3d1b5ae890f1c4a93d9d431addd442a1f16e9960/openlibrary/plugins/upstream/addbook.py#L179) function, which encodes a URL using [`encode_url_path`](https://github.com/internetarchive/openlibrary/blob/3d1b5ae890f1c4a93d9d431addd442a1f16e9960/openlibrary/plugins/upstream/addbook.py#L160) before passing it to `web.seeother`.

Maybe `encode_url_path` can be used by the `ReadableUrlProcessor` before any redirects?

#### Related files

Code changes likely only needed in this method:
https://github.com/internetarchive/openlibrary/blob/bee8ab4e369a500e1e31f73d3f06fb88e7c5896f/openlibrary/core/processors/readableurls.py#L40

#### Stakeholders

@cdrini

Start in openlibrary/core/processors/readableurls.py at the method identified in the issue, then reproduce the failing URL with its trailing slash and non-Latin slug. Compare its redirect handling with encode_url_path and safe_seeother in the linked addbook.py code; done means the example URL and equivalent author, edition, and work URLs load without an internal error.

Written by the indexing model from the issue text.