Merge issue #13120: Allow to call pdb.set_trace() from thread.

Patch by Ilya Sandler.

diff --cc Lib/pdb.py
Simple merge

diff --cc Lib/test/test_pdb.py
Simple merge

diff --cc Misc/NEWS
index c695be511ca1d8c6a95177df3c98a3f7777b2738,3894d2e3683a3ae29c653a5fea9fae0694920662..61270f435a4905baca7a3d8eeeb7adf8a33a87f4
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -104,9 -175,9 +104,12 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #13120: Allow to call pdb.set_trace() from thread.
+   Patch by Ilya Sandler.
+ 
 +- Issue #16585: Make CJK encoders support error handlers that return bytes per
 +  PEP 383.
 +
  - Issue #10182: The re module doesn't truncate indices to 32 bits anymore.
    Patch by Serhiy Storchaka.