You should check for DeprecationWarning in your code

When Python 2.7 was still supported, many functions were kept for backward

compatibility with Python 2.7. With the end of Python 2.7 support, these

backward compatibility layers have been removed, or will be removed soon.

Most of them emitted a :exc:`DeprecationWarning` warning for several years. For

example, using ``collections.Mapping`` instead of ``collections.abc.Mapping``

emits a :exc:`DeprecationWarning` since Python 3.3, released in 2012.

Test your application with the :option:`-W` ``default`` command-line option to see

:exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, or even with

:option:`-W` ``error`` to treat them as errors. :ref:`Warnings Filter

<warning-filter>` can be used to ignore warnings from third-party code.

It has been decided to keep a few backward compatibility layers for one last

release, to give more time to Python projects maintainers to organize the

removal of the Python 2 support and add support for Python 3.9.

Aliases to ref:`Abstract Base Classes <collections-abstract-base-classes>` in

the :mod:`collections` module, like ``collections.Mapping`` alias to

:class:`collections.abc.Mapping`, are kept for one last release for backward

compatibility. They will be removed from Python 3.10.

More generally, try to run your tests in the :ref:`Python Development Mode

<devmode>` which helps to prepare your code to make it compatible with the

next Python version.