Fix _addons/ URL prefix not being removed on Windows systems

Anki 2.1.28 Beta
https://forums.ankiweb.net/t/anki-2-1-28-beta/629/26

> Did the requirements on how to expose resources on the internal server change? Because scripts, css files and the like, registered with setWebExports, do not work anymore with beta2.
> A look at the console shows the following for all resources:
> Failed to load resource: the server responded with a status of 500 (INTERNAL SERVER ERROR)

**\Anki2\addons21\someaddon\__init__.py**
```py
from aqt import mw
import aqt

# assuming add-on folder is named "test"
# and assuming a 'test.js' is in the same folder
addon_id = "someaddon"
port = mw.mediaServer.getPort()
mw.addonManager.setWebExports(addon_id, ".*\\.js$")

print("Hello")

aqt.editor._html += f"""
<script>
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = 'http://127.0.0.1:{port}/_addons/{addon_id}/test.js';
    document.body.appendChild(script);
</script>"""
```

qt/aqt/mediasrv.py

@@ -179,7 +179,7 @@ def _redirectWebExports(path):
             return _exportFolder, addonPath
 
         try:
-            addon, subPath = addonPath.split(os.path.sep, 1)
+            addon, subPath = addonPath.split("/", 1)
         except ValueError:
             return addMgr.addonsFolder(), path
         if not addon: