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='d7e73930c004c902f78cb4b80eba88d3c057c4b8'/><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=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>refs</a><a class='active' href='/guix/log/gnu/build/bootloader.scm'>log</a><a href='/guix/tree/gnu/build/bootloader.scm?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>tree</a><a href='/guix/commit/gnu/build/bootloader.scm?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>commit</a><a href='/guix/diff/gnu/build/bootloader.scm?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/build/bootloader.scm'>
<input type='hidden' name='id' value='d7e73930c004c902f78cb4b80eba88d3c057c4b8'/><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=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>root</a>/<a href='/guix/log/gnu?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>gnu</a>/<a href='/guix/log/gnu/build?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>build</a>/<a href='/guix/log/gnu/build/bootloader.scm?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8'>bootloader.scm</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/gnu/build/bootloader.scm?id=d7e73930c004c902f78cb4b80eba88d3c057c4b8&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-11-03 07:59:59 +0100'>2020-11-03</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=a38d861e571c952f78ca588b50bdcc4adcb0c88c'>system: reconfigure: Use the disk-installer if provided.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-10-05 11:10:35 +0200'>2020-10-05</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=b97b423e3f61c80d5877dadc95b3f316cd61788f'>bootloader: Fix u-boot installation.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>