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 href='/guix/'>guix</a></td><td class='form'><form method='get'>
<input type='hidden' name='id' value='3f90245d91f07ab0c9672e7f38f08f31c107f0e4'/><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=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>refs</a><a class='active' href='/guix/log/po/doc/guix-manual.fa.po'>log</a><a href='/guix/tree/po/doc/guix-manual.fa.po?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>tree</a><a href='/guix/commit/po/doc/guix-manual.fa.po?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>commit</a><a href='/guix/diff/po/doc/guix-manual.fa.po?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/po/doc/guix-manual.fa.po'>
<input type='hidden' name='id' value='3f90245d91f07ab0c9672e7f38f08f31c107f0e4'/><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=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>root</a>/<a href='/guix/log/po?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>po</a>/<a href='/guix/log/po/doc?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>doc</a>/<a href='/guix/log/po/doc/guix-manual.fa.po?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4'>guix-manual.fa.po</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/po/doc/guix-manual.fa.po?id=3f90245d91f07ab0c9672e7f38f08f31c107f0e4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-04-18 13:18:17 +0200'>2021-04-18</span></td><td><a href='/guix/commit/po/doc/guix-manual.fa.po?id=43fb84da6749a78d12ac0f13db4cecb435de7903'>nls: Add Persian translation.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/packages/fa.po: New file.
* po/packages/LINGUAS: Add 'fa'.
* po/doc/guix-cookbook.fa.po: New file.
* po/doc/guix-manual.fa.po: New file.
* po/doc/local.mk (DOC_PO_FILES, DOC_COOKBOOK_PO_FILES): Add them.
* doc/local.mk (info_TEXINFOS): Add them.
</span></span></td><td>Julien Lepiller</td></tr>