Pin git python to <3.1.19

[patrickvonplaten]

What does this PR do?

GitPython did a new release which breaks our tests: https://github.com/gitpython-developers/GitPython/pull/1275/files
See: https://app.circleci.com/pipelines/github/huggingface/transformers/26010/workflows/a72a068e-b3f0-42e1-b08b-7e2c89cae3ed/jobs/245943 for example.

Pinning GitPython for now to make circle ci work

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[patrickvonplaten]

cc @LysandreJik @sgugger

Also see: gitpython-developers/GitPython#1296