[Bug]: Drop sentencepiece dependency

Describe the bug

sentencepiece seems no longer maintained--the last release was Feb 19, 2024-- and doesn't build on Python 3.13 on several platforms/configurations (e.g. macOS):

• Particular failure I had: https://github.com/google/sentencepiece/issues/1103
• Bug tracker has lots of issues with 3.13 build failures that have gone unaddressed: https://github.com/google/sentencepiece/issues

Since the package seems unmaintained, could flair drop the [sentencepiece] part of its transformers[sentencepiece] dependency so flair itself can build on macOS with Python 3.13?

To Reproduce

Build the latest sentencepiece (0.2.0) on e.g. macOS usng Python 3.13.

Expected behavior

flair builds on macOS with Python 3.13.

Logs and Stack traces

Screenshots

No response

Additional Context

No response

Environment

(P.S. The collect_env.py link above is wrong, it's missing the repo part of the URL.)

#### Versions:
##### Flair
0.15.0
##### Pytorch
2.5.1
##### Transformers
4.47.1
#### GPU
False