Switch to collections.abc.*

The abstract base classes previously defined in 'collections' were moved
to 'collections.abc' in 3.3. The aliases will be removed in 3.10.
Preempt this change now with a simple find-replace:

  $ ag -l 'collections.($TYPES)' | \
      xargs sed -i 's/\(collections\)\.\($TYPES\)/\1.abc.\2/g'

Where $TYPES is the list of moved ABCs from [1].

[1] https://docs.python.org/3/library/collections.abc.html

Credit for this commit message to Stephen Finucane <stephenfin@redhat.com>

Change-Id: I782f2449832398cf2cfb886f561635de3d8ac18d

troveclient/compat/tests/test_common.py

@@ -254,7 +254,7 @@ class CommandsBaseTest(testtools.TestCase):
 
         self.cmd_base.verbose = False
 
-        class MockIterable(collections.Iterable):
+        class MockIterable(collections.abc.Iterable):
             links = ["item"]
             count = 1