buildserver: use py3's pathlib to generate cache file URL

This should handle edge cases better, like odd characters in the path, etc.

makebuildserver

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import os
+import pathlib
 import requests
 import stat
 import sys
@@ -52,9 +53,8 @@ options, args = parser.parse_args()
 cachedir = os.path.join(os.getenv('HOME'), '.cache', 'fdroidserver')
 config = {
     'basebox': 'jessie64',
-    # TODO in py3, convert this to pathlib.Path(absolute_path_string).as_uri()
     'baseboxurl': [
-        'file://' + os.path.join(cachedir, 'jessie64.box'),
+        pathlib.Path(os.path.join(cachedir, 'jessie64.box')).as_uri(),
         'https://f-droid.org/jessie64.box',
     ],
     'debian_mirror': 'http://http.debian.net/debian/',