replace Python-base tokenizer by non-fast tokenizer in error message

[SaulLu]

What does this PR do?

As one user rightly pointed out in an issue #17809, when a user receives the error "tokens() is not available when using Python-based tokenizers" it is not obvious that a python-based tokenizer refers to a tokenizer class without the term Fast at the end.

I therefore propose to change the error messages using this term to refer to the term fast which is more easily understood by users.

Fixes #17809

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Who can review?

Anyone in the community is free to review the PR once the tests have passed. Would love to have the approval of @sgugger , @LysandreJik , @patrickvonplaten or @patil-suraj 🤗

[HuggingFaceDocBuilderDev]

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