Install only the needed lint tool in style checks CI

[weiji14]

Instead of installing all of torchgeo's required dependencies, just install the lint tool (black/flake8/isort/pydocstyle/pyupgrade) directly.

Should make for a faster style checks CI. From 1m50s to 32s. Compare https://github.com/microsoft/torchgeo/actions/runs/2367868140 vs https://github.com/microsoft/torchgeo/actions/runs/2437296402

As per #552 (comment)

[adamjstewart]

One downside of this, and I think the reason I didn't do this originally, is that it becomes yet another place we need to list our dependencies. We can't easily manage these dependencies with dependabot, and we won't be able to test minimal version compatibility. Of course, these deps are much less critical for users, so the faster tests may outweigh the centralized dep management.

[adamjstewart]

I'm investigating dependabot now. If it lets us have multiple files, e.g.:

• requirements/requirements.txt (defaults)
• requirements/requirements-style.txt (style tests)
• requirements/requirements-tests.txt (pytest, mypy)

we may be able to get the best of both worlds.

[adamjstewart]

Update: it doesn't let us have multiple files. I wonder if we can grep the specific version requirements we need from a single requirements.txt so that we don't install the latest version of things.

[weiji14]

Closing as supersed by changes in #551