Update install dependencies

[IgorSusmelj]

Changes

• removes openapi.txt requirements file
• move openapi dependencies to base.txt
• use restore keys (apparently this should speed up the tests as we can use the cache from the master branch when creating PRs!?)
• added a test to check minimal dependencies so we can track whenever we add new things that require version updates
• update the readme and remove support for Python 3.6

This allows to install the lightly package with minimal dependencies (no PyTorch and torchvision)

New minimal dependencies (to run all the unittests) are the following:

pytorch_lightning==1.7.1
torch==1.11.0
torchvision==0.12.0

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2b215aa) 85.50% compared to head (3d173f6) 85.50%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1476 +/- ## ======================================= Coverage 85.50% 85.50% ======================================= Files 135 135 Lines 5657 5657 ======================================= Hits 4837 4837 Misses 820 820 ```

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