use common var for 'config.yml', standarize on UTF-8

This makes it easy to track all the places that use config.yml, and
hopefully makes things feel cleaner.  This also standardizes all places
where config.yml is written out to use UTF-8 as the file encoding.

This also includes a lot of black code format fixes.

tests/test_publish.py

@@ -18,7 +18,6 @@ import shutil
 import sys
 import unittest
 import tempfile
-from pathlib import Path
 from unittest import mock
 
 from fdroidserver import publish
@@ -96,7 +95,7 @@ class PublishTest(unittest.TestCase):
         ]
 
         os.chdir(self.testdir)
-        Path('config.yml').write_text('')
+        common.write_config_file('')
 
         publish.store_stats_fdroid_signing_key_fingerprints(appids, indent=2)
 
@@ -118,7 +117,7 @@ class PublishTest(unittest.TestCase):
         self.assertEqual(expected, common.load_stats_fdroid_signing_key_fingerprints())
 
         yaml = ruamel.yaml.YAML(typ='safe')
-        with open('config.yml') as fp:
+        with open(common.CONFIG_FILE) as fp:
             config = yaml.load(fp)
         self.assertEqual(
             'c58460800c7b250a619c30c13b07b7359a43e5af71a4352d86c58ae18c9f6d41',