Handle different release date formats

Some release entries in the MusicBrainz database have different formats
for the date field. In particular, a few have only a year value, while
others have a year and month. This commit introduces a new `parse_date`
function that attempts to parse a string into a date using one of the
known formats. The `release_year` function uses this new function to
parse the date and extract the year, but now also returns `None` if the
date could not be parsed or was not present in the record.

ripcd.py

@@ -28,6 +28,13 @@ log = logging.getLogger('ripcd2')
 filesystemencoding = sys.getfilesystemencoding()
 
 
+DATE_FMTS = [
+    '%Y-%m-%d',
+    '%Y-%m',
+    '%Y',
+]
+
+
 DRIVE_OFFSETS = {
     'ASUS_BW-12B1ST_a': 6,
     'ATAPI_iHES212_3': 702,
@@ -254,6 +261,16 @@ def get_release_list_from_device(device):
         return res['release-list']
 
 
+def parse_date(datestr):
+    for fmt in DATE_FMTS:
+        try:
+            return datetime.datetime.strptime(datestr, fmt)
+        except ValueError:
+            continue
+    else:
+        raise ValueError('Could not parse date string "{}"'.format(datestr))
+
+
 def prompt(text, validate=None):
     while True:
         try:
@@ -299,7 +316,10 @@ def prompt_select_release(release_list):
 
 
 def release_year(release):
-    return datetime.datetime.strptime(release['date'], '%Y-%m-%d').year
+    try:
+        return parse_date(release['date']).year
+    except (KeyError, ValueError):
+        return None
 
 
 def safe_name(name):