Drop Python 3.6 support

[akihironitta]

What does this PR do?

1.4.8 Dropping Python 3.6 support should fix the failing tests as commented in #529 (comment).

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[Borda]

as chatted, lest rather bump the python version to 3.7 than set too tight PL version range...

[akihironitta]

Once the running CI is all green, this PR should be ready for getting merged.

UPDATE: All runs are successful 🎉