TOOLS/matroska.py: fix some minor things for dumping

TOOLS/matroska.py can be used stand-alone for dumping Matroska file
contents. Fix some related issues.

Elements were treated as unknown if the element hex ID contained
uppercase characters.

Unknown elements stopped parsing. This was intentional, but I don't
really see any reason for it.

Dumping with Python 2 is broken. I don't care, but everytime I hit this,
I find myself trying to find out why. So make it error out explicitly.

TOOLS/matroska.py

@@ -271,6 +271,7 @@ def parse_elems(l, namespace):
     for el in l:
         if isinstance(el, str):
             name, hexid, eltype = [x.strip() for x in el.split(',')]
+            hexid = hexid.lower()
             multiple = name.endswith('*')
             name = name.strip('*')
             new = MatroskaElement(name, hexid, eltype, namespace)
@@ -400,10 +401,6 @@ def read_float(s, length):
 def parse_one(s, depth, parent, maxlen):
     elid = hexlify(read_id(s)).decode('ascii')
     elem = elementd.get(elid)
-    if parent is not None and elid not in parent.subids and elid not in ('ec', 'bf'):
-        print('Unexpected:', elid)
-        if 1:
-            raise NotImplementedError
     size, length = read_vint(s)
     this_length = len(elid) / 2 + size + length
     if elem is not None:
@@ -442,7 +439,7 @@ def parse_one(s, depth, parent, maxlen):
         else:
             raise NotImplementedError
     else:
-        print(depth, 'Unknown element:', elid, 'size:', length)
+        print("    " * depth, '[' + elid + '] Unknown element! size:', length)
         read(s, length)
     return this_length
 
@@ -455,6 +452,8 @@ if __name__ == "__main__":
     elif sys.argv[1] == '--generate-definitions':
         generate_C_definitions(sys.stdout)
     else:
+        if sys.version_info.major < 3:
+            raise Exception("Dumping requires Python 3.")
         s = open(sys.argv[1], "rb")
         while 1:
             start = s.tell()