chore(python): update .kokoro/requirements.txt

[parthea]

The release script failed in this log with

Collecting jaraco.classes
ERROR: In --require-hashes mode, all requirements must have their versions pinned with ==. These do not:
    jaraco.classes from [https://files.pythonhosted.org/packages/8f/e6/f92d21f915cee7acf1cfe3e0ec60b8e9888dcf40a9814e3838a87ba487d0/jaraco.classes-3.2.2-py3-none-any.whl#sha256=e6ef6fd3fcf4579a7a019d87d1e56a883f4e4c35cfe925f86731abc58804e647](https://www.google.com/url?q=https://files.pythonhosted.org/packages/8f/e6/f92d21f915cee7acf1cfe3e0ec60b8e9888dcf40a9814e3838a87ba487d0/jaraco.classes-3.2.2-py3-none-any.whl%23sha256%3De6ef6fd3fcf4579a7a019d87d1e56a883f4e4c35cfe925f86731abc58804e647&sa=D) (from keyring==23.9.0->-r github/python-bigquery-data-exchange/.kokoro/requirements.txt (line 254))

I believe the issue is that the requirements.txt file is out of date and doesn't include jaraco.classes

[parthea]

⚡ Thank you @dandhlee for the lightning fast review!