Issue #20766: Merge with 3.6.

diff --cc Misc/NEWS
index 0ffbefc6ecba32d2de2fcdcce841805bbc485ea6,667bff5eff09b7bebccef67d24bacec5201c5301..ff8c8f293c591bbe03f6f4e7c76425a19a30c7e2
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -85,6 -101,6 +85,9 @@@ Core and Builtin
  Library
  -------
  
++- Issue #20766: Fix references leaked by pdb in the handling of SIGINT
++  handlers.
++
  - Issue #27998: Fixed bytes path support in os.scandir() on Windows.
    Patch by Eryk Sun.