[utils] Support ttaf1 namespace in TTML

It's found in bbc.co.uk. See #6038

youtube_dl/utils.py

@@ -1841,7 +1841,10 @@ def srt_subtitles_timecode(seconds):
 
 
 def dfxp2srt(dfxp_data):
-    _x = functools.partial(xpath_with_ns, ns_map={'ttml': 'http://www.w3.org/ns/ttml'})
+    _x = functools.partial(xpath_with_ns, ns_map={
+        'ttml': 'http://www.w3.org/ns/ttml',
+        'ttaf1': 'http://www.w3.org/2006/10/ttaf1',
+    })
 
     def parse_node(node):
         str_or_empty = functools.partial(str_or_none, default='')
@@ -1849,9 +1852,9 @@ def dfxp2srt(dfxp_data):
         out = str_or_empty(node.text)
 
         for child in node:
-            if child.tag in (_x('ttml:br'), 'br'):
+            if child.tag in (_x('ttml:br'), _x('ttaf1:br'), 'br'):
                 out += '\n' + str_or_empty(child.tail)
-            elif child.tag in (_x('ttml:span'), 'span'):
+            elif child.tag in (_x('ttml:span'), _x('ttaf1:span'), 'span'):
                 out += str_or_empty(parse_node(child))
             else:
                 out += str_or_empty(xml.etree.ElementTree.tostring(child))
@@ -1860,7 +1863,7 @@ def dfxp2srt(dfxp_data):
 
     dfxp = xml.etree.ElementTree.fromstring(dfxp_data.encode('utf-8'))
     out = []
-    paras = dfxp.findall(_x('.//ttml:p')) or dfxp.findall('.//p')
+    paras = dfxp.findall(_x('.//ttml:p')) or dfxp.findall(_x('.//ttaf1:p')) or dfxp.findall('.//p')
 
     if not paras:
         raise ValueError('Invalid dfxp/TTML subtitle')