python-pickleshare is not compatible with Python 3.9

While it still builds, packages that use its code, like ipykernel fail to build from source.

This currently blocks the initial Python 3.9 bootstrap sequence.

The code that requires fixing:

diff --git a/pickleshare.py b/pickleshare.py
index 31f093c..a7e5e76 100644
--- a/pickleshare.py
+++ b/pickleshare.py
@@ -45,7 +45,7 @@ except ImportError:
     from pathlib2 import Path
 
 import os,stat,time
-import collections
+import collections.abc
 try:
     import cPickle as pickle
 except ImportError:
@@ -63,7 +63,7 @@ def gethashfile(key):
 
 _sentinel = object()
 
-class PickleShareDB(collections.MutableMapping):
+class PickleShareDB(collections.abc.MutableMapping):
     """ The main 'connection' object for PickleShare database """
     def __init__(self,root):
         """ Return a db object that will manage the specied directory"""