[llvm][utils] Remove Python2 comaptaible import in unicode-case-fold.py

These imports got moved around in Python 3.0
(https://docs.python.org/3/whatsnew/3.0.html#library-changes).

LLVM requires Python >= 3.8 so we can assume the Python3 names
are available.

llvm/utils/unicode-case-fold.py

@@ -21,11 +21,7 @@ from __future__ import print_function
 
 import sys
 import re
-
-try:
-    from urllib.request import urlopen
-except ImportError:
-    from urllib2 import urlopen
+from urllib.request import urlopen
 
 
 # This variable will body of the mappings function