[opt-viewer] Listify `dict_items` for Py3 indexing

[modocache]

Summary

In Python 2, calling dict.items() returns an indexable list, whereas
on Python 3 it returns a set-like dict_items object, which cannot be
indexed. Explicitly onvert the dict_items object so that it can be
indexed when using Python 3.

In combination with D36622, D36623, and D36624, this change allows
opt-viewer.py to exit successfully when run with Python 3.4.

Test Plan:
Run opt-viewer.py using Python 3.4 and confirm it does not encounter a
runtime error when when indexing into dict.items().

[anemet]

LGTM, thanks.

[modocache]

Great, thanks!

tools/opt-viewer/optrecord.py

class Remark(yaml.YAMLObject):
 
     def getArgString(self, mapping):
         mapping = dict(list(mapping))
         dl = mapping.get('DebugLoc')
         if dl:
             del mapping['DebugLoc']
 
         assert(len(mapping) == 1)
-        (key, value) = mapping.items()[0]
+        (key, value) = list(mapping.items())[0]
 
         if key == 'Caller' or key == 'Callee':
             value = cgi.escape(demangle(value))
 
         if dl and key != 'Caller':
             dl_dict = dict(list(dl))
             return "<a href={}>{}</a>".format(
                 make_link(dl_dict['File'], dl_dict['Line']), value)