tensorflow_docs requires python3.9

[nicolaspi]

Description

Brief Description of the PR:

The CI is failing with the error (e.g. #2739):

#20 2.103   File "/usr/local/lib/python3.7/site-packages/tensorflow_docs/api_generator/reference_resolver.py", line 88, in ReferenceResolver
#20 2.103     physical_path: Optional[dict[str, str]] = None,
#20 2.103 TypeError: 'type' object is not subscriptable

because tensorflow_docs is using a feature introduced in python3.9. This fix bumps the python version for the docs_tests target in tools/docker/sanity_check.Dockerfile.

Type of change

• Bug fix

How Has This Been Tested?

Locally the command bash tools/run_build.sh run successfully after the change.

[seanpmorgan]

Thanks @nicolaspi !