Not Windows-compatible code in loggers

**Describe the bug**

In https://github.com/pawamoy/mkdocstrings/blob/master/src/mkdocstrings/loggers.py#L107:

```python
return template.filename.rsplit("/templates/", 1)[1]
```

The forward slashes are not cross-platform :wink:

**Additional context**
We should probably use `os.sep` instead of `/` here.
