Deprecation notice for collection.Sequence in recurrent.py

[ulf1]

@qlzh727

Describe the current behavior

When running unit tests the following deprecation note appears

lib/python3.7/site-packages/tensorflow_core/python/keras/layers/recurrent.py:808: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
  if (isinstance(inputs, collections.Sequence)

Describe the expected behavior

No deprecation notice

Line 808 needs a minor modification

...
  if (isinstance(inputs, collections.abc.Sequence)
...

See python 3 docs

[qlzh727]

Since we still have to support py2 within Google for now, we can't migrate to py3 only code yet.

[tensorflow-bot]

Are you satisfied with the resolution of your issue?
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[bjourne]

Looks like this has been fixed the proper way now:

try:
  from collections import abc as collections_abc  # pylint: disable=g-import-not-at-top
except ImportError:  # For Python 2
  import collections as collections_abc  # pylint: disable=g-import-not-at-top