Raise minimum supported Python version to 3.8

[gabrielfruet]

Summary

Raise Lightly's minimum supported Python version to 3.8.

Why

Python 3.7 is EOL

Scope

• packaging metadata/classifiers
• CI workflow matrices and helper targets
• docs / Read the Docs references

Acceptance

• minimum version is 3.8+
• CI no longer runs 3.7
• docs and metadata match

[Borda]

@liopeer shall it be 3.10 since all lower versions are already with ended support, end of the lifecycle?

[gabrielfruet]

Hi @Borda,

We had an internal discussion. Since some of our customers still run old clusters with older GPUs, and support for 3.7, 3.8, or 3.10 isn’t blocking us, we’ve decided to drop Python 3.7 (because it's annoying to support it 😅) but keep 3.8, most of those older GPUs still support the py38 + torch combination.