smartplaylist: change encoding of additional field

URL-encode additional item fields within generated EXTM3U playlists instead of JSON-encoding them.
This is because JSON-encoding additional fields/attributes made it difficult to parse the `EXTINF` line but using URL-encoding for these values makes parsing easy (because URL-encoded values cannot contain quotation marks and spaces).

I introduced the generation of additional EXTM3U item fields earlier this year and I want to make it right now.

**Design/definition background:**
Unfortunately, I didn't find a clear definition of how additional playlist item attributes should be encoded - apparently there is none.
Given that item URIs within an M3U playlist can be URL-encoded already, defining the values of additional attributes to be URL-encoded is consistent design.
I didn't find examples of additional M3U item attributes in the web where the attribute value contains a space or quotation mark but examples that specified numeric IDs and URLs as attribute values.
Because the URL attribute examples I found didn't contain URL-encoded characters and because it is more readable and unproblematic for parsing, I've let the attribute URL encoding treat `:` and `/` as safe characters.

**Breaking change:**
While this is a breaking change in theory, in practice it is not since afaik all integrations of the smartplaylist plugin's additional EXTM3U item attribute generation feature (beets-webm3u) work with simple attribute values such as the item ID (numeric) whose formatting/encoding is not affected when changing from JSON to URL-encoding.
In other words the change is backward-compatible with the beets-webm3u plugin (which I'll adjust correspondingly after this beets PR was merged).

beetsplug/smartplaylist.py

@@ -14,9 +14,9 @@
 
 """Generates smart playlists based on beets queries."""
 
-import json
 import os
 from urllib.request import pathname2url
+from urllib.parse import quote
 
 from beets import ui
 from beets.dbcore import OrQuery
@@ -327,7 +327,7 @@ def update_playlists(self, lib, pretend=False):
                         if extm3u:
                             attr = [(k, entry.item[k]) for k in keys]
                             al = [
-                                f" {a[0]}={json.dumps(str(a[1]))}" for a in attr
+                                f" {a[0]}={quote(str(a[1]), safe='/:')}" for a in attr
                             ]
                             attrs = "".join(al)
                             comment = "#EXTINF:{}{},{} - {}\n".format(

test/plugins/test_smartplaylist.py

@@ -283,7 +283,7 @@ def test_playlist_update_output_extm3u_fields(self):
         assert (
             content
             == b"#EXTM3U\n"
-            + b'#EXTINF:300 id="456" genre="Fake Genre",Fake Artist - fake Title\n'
+            + b'#EXTINF:300 id="456" genre="Fake%20Genre",Fake Artist - fake Title\n'
             + b"/tagada.mp3\n"
         )