Background (read
Python 3 documentation for the collections module
for more detail):

• Collections Abstract Base Classes
  ("CABCs") moved to the collections.abc module in Python 3.3.

• These "Collections Abstract Base Classes" are: AsyncGenerator,
  AsyncIterable, AsyncIterator, Awaitable, Bytestring,
  Callable, Collection, Container, Coroutine, Generator,
  Hashable, ItemsView, Iterable, Iterator, KeysView,
  Mapping, MappingView, MutableMapping, MutableSequence,
  MutableSet, Reversible, Sequence, Set, Sized, ValuesView.

• For backwards compatibility, CABCs continue to be visible
  in the collections module through Python 3.8; and can be
  imported either from collections or from collections.abc.

• Starting with Python 3.9, CABCs must be imported from collections.abc.

• In Python 2 they could only be imported from 'collections',
  not from 'collections.abc'. So while Sage was keeping
  Py2 and Py3 compatible it made sense to import from collections.

• In Python 3.7 and Python 3.8, importing them from collections
  gives a deprecation warning. This warning was silenced in Sage,
  see py3: last test in tests/cmdline.py #28002.

In this ticket we import the classes from collections.abc.

We do not suppress the silencing of the deprecation warning yet.
See comment:23 and #30768.

CC: @fchapoton @jhpalmieri @slel @videlec @tscrim

Component: python3

Keywords: days101, collections.abc

Author: Samuel Lelièvre

Branch: 4f80136

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/28012