Be compatible with python-werkzeug 1.0.0.

Taken from upstream:
https://github.com/beetbox/beets/commit/d43d54e21cde97f57f19486925ab56b419254cc8

diff --git a/beetsplug/web/__init__.py b/beetsplug/web/__init__.py
index f53fb3a954..21ff5d94ed 100644
--- a/beetsplug/web/__init__.py
+++ b/beetsplug/web/__init__.py
@@ -169,7 +169,7 @@ def to_python(self, value):
         return ids
 
     def to_url(self, value):
-        return ','.join(value)
+        return ','.join(str(v) for v in value)
 
 
 class QueryConverter(PathConverter):
/a> : <a href='/guix/'>guix</a></td><td class='form'><form method='get'>
<input type='hidden' name='id' value='7937c8827b8d23347a3159b4696335bd19fc17aa'/><input type='hidden' name='showmsg' value='1'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=7937c8827b8d23347a3159b4696335bd19fc17aa'>refs</a><a class='active' href='/guix/log/tests/cve-sample.json?showmsg=1'>log</a><a href='/guix/tree/tests/cve-sample.json?id=7937c8827b8d23347a3159b4696335bd19fc17aa'>tree</a><a href='/guix/commit/tests/cve-sample.json?id=7937c8827b8d23347a3159b4696335bd19fc17aa'>commit</a><a href='/guix/diff/tests/cve-sample.json?id=7937c8827b8d23347a3159b4696335bd19fc17aa'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/cve-sample.json'>
<input type='hidden' name='id' value='7937c8827b8d23347a3159b4696335bd19fc17aa'/><input type='hidden' name='showmsg' value='1'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=7937c8827b8d23347a3159b4696335bd19fc17aa&amp;showmsg=1'>root</a>/<a href='/guix/log/tests?id=7937c8827b8d23347a3159b4696335bd19fc17aa&amp;showmsg=1'>tests</a>/<a href='/guix/log/tests/cve-sample.json?id=7937c8827b8d23347a3159b4696335bd19fc17aa&amp;showmsg=1'>cve-sample.json</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/cve-sample.json?id=7937c8827b8d23347a3159b4696335bd19fc17aa'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2020-12-21 17:47:35 +0100'>2020-12-21</span></td><td class='logsubject'><a href='/guix/commit/tests/cve-sample.json?id=9608f4003dedd8dfe99327c15668ca1a43ebd93b'>tests: Fix malformed JSON.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Guile-JSON 4.3.2 would parse in spite of these typos, but 4.4.1 is
stricter.

* tests/swh.scm (%directory-entries): Add missing comma.
* tests/cve-sample.json: Likewise.


</td></tr>