Pin version exclusion for tensorflow incompatible with keras
Once `tensorflow` version 2.6.2 is released:
- https://github.com/tensorflow/tensorflow/commit/c1867f3bfdd1042f694df7a9870be51ba80543cb
- https://pypi.org/project/tensorflow/2.6.2/

with the patch:
- tensorflow/tensorflow#52927

we can remove the temporary fix we introduced in:
- #3208

Fix #3209.