prefixed URIs with forward slash not rendered correctly

When a forward slash (escaped with a reserved character escape sequences) is part of a prefixed URI, for example:
```
doi:10.0000\/foobar
```
it is rendered as `nsX:foobar` with `X` a number. Replacing `\/' i.e. by `_` leads to a correct rendering.

Discovered using the fork https://github.com/fatestigma/ontology-visualization on Ubuntu 20.04, with Python 3.8.10, and python3-rdflib 4.2.2-5.