If this excess part isn't trimmed from the ID, it can cause unexpected
results when calling some API methods.
For example; this code now correctly returns all albums for an artist:
```
artist = 'https://open.spotify.com/artist/7oPftvlwr6VrsViSDV7fJY?si=M3PrzRC4TBOZu8YyLYc-tA'
artist_albums = sp.artist_albums(artist)
```
Previously, this code mimicked same behaviour as calling
`spotify.artist`, but now will return albums as expected
with this commit.
Fixes#365 and #323.
Ritiek Malhotra