Fix #957 KeyError when 'dur' not present in XML subtitles (#958)

* Fix #957 KeyError when 'dur' not present in XML subtitles by setting a default duration of 0.0 seconds

pytube/captions.py

@@ -67,7 +67,10 @@ class Caption:
         for i, child in enumerate(list(root)):
             text = child.text or ""
             caption = unescape(text.replace("\n", " ").replace("  ", " "),)
-            duration = float(child.attrib["dur"])
+            try:
+                duration = float(child.attrib["dur"])
+            except KeyError:
+                duration = 0.0
             start = float(child.attrib["start"])
             end = start + duration
             sequence_number = i + 1  # convert from 0-indexed to 1.