Add Python 3.9 and drop PyTorch 1.6

[akihironitta]

What does this PR do?

Follow up to #785.

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[Borda]

this is quite strange failer for python 3.9:

(783 durations < 0.005s hidden.  Use -vv to show these durations.)
===== 295 passed, 30 skipped, 1 xfailed, 699 warnings in 517.34s (0:08:37) =====
PyThreadState_Clear: warning: thread still has a frame
free(): invalid next size (fast)
/home/runner/work/_temp/db56bfec-2415-457e-bc8a-b3fbaa931722.sh: line 1:  4579 Aborted                 (core dumped) python -m pytest pl_bolts tests -v --cov=pl_bolts --junitxml=junit/test-results-Linux-3.9-minimal.xml

https://github.com/PyTorchLightning/lightning-bolts/runs/4575553319?check_suite_focus=true#step:10:839

[akihironitta]

Let me have a quick look.

Tried to find similar issues for 10 min but couldn't really find any...

EDIT: Some related issues are linked in pytest-dev/pytest#8841 (comment) but haven't had a closer look...