Add more meaningful comments about Python deprecations

Thanks to Bruno Alla (@browniebroke)

src/future/moves/_dummy_thread.py

@@ -2,6 +2,8 @@
 from future.utils import PY3
 
 if PY3:
+    # _dummy_thread and dummy_threading modules were both deprecated in
+    # Python 3.7 and removed in Python 3.9
     try:
         from _dummy_thread import *
     except ImportError:

tests/test_future/test_builtins.py

@@ -1307,6 +1307,9 @@ def test_pow(self):
         if sys.version_info[:2] < (3, 8):
             self.assertRaises((TypeError, ValueError), pow, -1, -2, 3)
         else:
+            # Changed in version 3.8: For int operands, the three-argument form
+            # of pow now allows the second argument to be negative, permitting
+            # computation of modular inverses.
             self.assertEqual(pow(-1, -2, 3), 1)
         self.assertRaises(ValueError, pow, 1, 2, 0)