[setup] correct transformers version format
setuptools has a pretty fixed expectation of version numbers.

```
x.y.z
x.y.z.dev0
x.y.z.rc1
```

This PR fixes the dev version number and adds a comment with correct formats for the future editors

This fix removes this warning on `make fixup|style|etc` or any other time `setup.py` is being run.
```
setuptools/dist.py:452: UserWarning: Normalizing '4.2.0dev0' to '4.2.0.dev0'
  warnings.warn(tmpl.format(**locals()))
```
and the alternative:
```
/setuptools/dist.py:452: UserWarning: Normalizing '4.0.0-rc-1' to '4.0.0rc1'
```

Fixes: #8749

@LysandreJik, @sgugger
