standardize on .yml as the file extension for YAML

Though the YAML people recommend .yaml for the file extension, in Android
land it seems clear that .yml has won out:

* .travis.yml
* .gitlab-ci.yml
* .circle.yml
* Ansible main.yml

tests/metadata.TestCase

@@ -33,7 +33,7 @@ class MetadataTest(unittest.TestCase):
         config = dict()
         config['sdk_path'] = '/opt/android-sdk'
         config['ndk_paths'] = dict()
-        config['accepted_formats'] = ['json', 'txt', 'xml', 'yaml']
+        config['accepted_formats'] = ['json', 'txt', 'xml', 'yml']
         fdroidserver.common.config = config
 
         apps = fdroidserver.metadata.read_metadata(xref=True)

tests/metadata/org.videolan.vlc.pickle

@@ -4618,7 +4618,7 @@ sasS'FlattrID'
 p1324
 NsS'metadatapath'
 p1325
-S'metadata/org.videolan.vlc.yaml'
+S'metadata/org.videolan.vlc.yml'
 p1326
 sS'Disabled'
 p1327

tests/run-tests

@@ -163,7 +163,7 @@ cp $WORKSPACE/tests/metadata/org.smssecure.smssecure.txt $REPOROOT/metadata/
 $fdroid readmeta
 
 # now make a fake duplicate
-touch $REPOROOT/metadata/org.smssecure.smssecure.yaml
+touch $REPOROOT/metadata/org.smssecure.smssecure.yml
 
 set +e
 $fdroid readmeta