use transformers sentencepiece requirement

[helpmefindaname]

fixes #2833

As flair doesn't use sentencepiece itself, but lets transformers use it indirectly, I propose to let the transformers package handle that dependency.
This has two advantages:
a) we can use a more broad range of sentencepiece versions
b) if there are any new versions that break something, the transformers guys will handle it

Note: relies on #2827 to be merged first for passing tests

[whoisjones]

👍

[alanakbik]

Makes a lot of sense, thanks @helpmefindaname!