Fix conda release

[lhoestq]

There were a few issues with conda releases (they've been failing for a while now).
To fix this I had to:

• add the --single-version-externally-managed tag to the build stage (suggestion from here)
• set the python version of the conda build stage to 3.8 since 3.9 isn't supported
• sync the evrsion requirement of huggingface_hub

With these changes I'm working on uploading all missing versions until 1.6.2 to conda

EDIT: I managed to build and upload all missing versions until 1.6.2 to conda :)