[opt-viewer] Use Python 3-compatible `intern()`

[modocache]

Summary

In Python 2, intern() is a builtin function available to all programs.
In Python 3, it was moved into the sys module, available as
sys.intern. Import it such that, within optrecord.py, intern() is
available whether run using Python 2 or 3.

Test Plan:
Run opt-viewer.py using Python 3, confirm it no longer
encounters a runtime error when intern() is called.

[anemet]

LGTM, assuming it still works with Python2. (I don't see why it wouldn't.)

[modocache]

Yup, I've confirmed this still works with Python 2. Thanks, @anemet!

tools/opt-viewer/optrecord.py

 
 import cgi
 from collections import defaultdict
 import fnmatch
 import functools
 from multiprocessing import Lock
 import os, os.path
 import subprocess
+try:
+    # The previously builtin function `intern()` was moved
+    # to the `sys` module in Python 3.
+    from sys import intern
+except:
+    pass
 
 import optpmap
 
 
 p = subprocess.Popen(['c++filt', '-n'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
 p_lock = Lock()