Improve GitHub Action for testing

[cthoyt]

As I'm trying to figure out why #266 isn't working, I want to try adding a couple less controversial things to the existing testing workflow

• [x] Test on more modern versions of Python (3.6 has passed its end of life as of December 2021)
• [x] Test directly with pytest instead of setup.py (the Python Packge Authority has deprecated all usages of setup.py as a script
• [x] Add torch to install_requires in the setup.py to signify that this package requires torch after it's installed. You can always install a specific version of torch before trying to install torch-scatter and it will respect what's already available in the environment.

Things that didn't work:

1. Testing on Mac OS~ (this doesn't work without getting rid of the +cpu)
2. Removing the -e (editable mode) flag causes ImportError: Could not find module '_version_cpu' in /home/runner/work/pytorch_scatter/pytorch_scatter/torch_scatter
3. Adding torch to setup_requires in the setup.py is a bit of a Catch-22: you already have to be executing the setup.py to recognize that. Getting this right can probably be done with the declarative setup that I proposed in #266, but that one still needs a few things to be clarified (plus CI to pass)

[codecov-commenter]

Codecov Report

Merging #268 (15af45a) into master (fc1b139) will decrease coverage by 0.49%. The diff coverage is n/a.

:exclamation: Current head 15af45a differs from pull request most recent head 18679be. Consider uploading reports for the commit 18679be to get more accurate results

@@            Coverage Diff             @@
##           master     #268      +/-   ##
==========================================
- Coverage   97.54%   97.05%   -0.50%     
==========================================
  Files           9        9              
  Lines         204      204              
==========================================
- Hits          199      198       -1     
- Misses          5        6       +1     

Impacted Files	Coverage Δ
torch_scatter/scatter.py	96.00% <0.00%> (-2.00%)	:arrow_down:

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[cthoyt]

@rusty1s this is a bit more baked than #266 and has a few notes on what works and what doesn't - it's ready for review if you've got some time :)

[github-actions[bot]]

This pull request had no activity for 6 months. It will be closed in 2 weeks unless there is some new activity.

[github-actions[bot]]

This pull request had no activity for 6 months. It will be closed in 2 weeks unless there is some new activity.

[github-actions[bot]]

This pull request had no activity for 6 months. It will be closed in 2 weeks unless there is some new activity.