use set literals and generators in gdb scripst

Summary: Presume Python 3 v.s. attempting to support all versions of Python.

Differential Revision: D34232485

fbshipit-source-id: 6a8e4755b1cac750dc30c4d6f678c99d05da7deb

folly/experimental/gdb/deadlock.py

@@ -166,7 +166,7 @@ def simple_cycles(G):  # noqa: C901
     """
 
     def _unblock(thisnode, blocked, B):
-        stack = set([thisnode])
+        stack = {thisnode}
         while stack:
             node = stack.pop()
             if node in blocked:

folly/fibers/scripts/gdb.py

@@ -905,7 +905,7 @@ def get_fiber_manager_map_vevb():
 # reset the caches when we continue; the current fibers will almost certainly change
 def clear_fiber_caches(*args):
     # default to only clearing manager and info caches
-    caches = set(arg.string() for arg in args) or {"managers", "info"}
+    caches = {arg.string() for arg in args} or {"managers", "info"}
     cleared = set()
     if "managers" in caches:
         cleared.add("manager")