Use python 3.8 in CI builds

[breznak]

All PRs CI use py3.8, for releases we provide 3.7, 3.8.

Fixes #734 #809

[breznak]

linux py3.8 tests fail,
https://github.com/htm-community/htm.core/pull/820/checks?check_run_id=731129639#step:9:1053

strangely, I cannot replicate that locally on 20.04.

[breznak]

I don't know why tests would start failing to import on Py3.8

[breznak]

@dkeeney this is now passing, please review Py3.8 support.

[dkeeney]

Ok, I understand what the change is. I centers the expected result better within the 0.003 allowance.
Did you confirm it did not break 3.7?

[breznak]

centers the expected result better within the 0.003 allowance.

this is just a fix for the prev merged PR for RDSE params_check(), which has random behavior - and on OSX I randomly failed the assert. As adviced, I increased the SDR size (so conflicts are more rare) and that changed the results.

Did you confirm it did not break 3.7?

The test didn't. But now we fully test only on py3.8. Do we want to keep some platform on 3.7 to cover both? Good idea.

[dkeeney]

Will, even if you manually checked it with 3.7 it would be ok.

[breznak]

Do we want to keep some platform on 3.7 to cover both?

@dkeeney merged as is now, so we run everything on py3.8, do we want to keep, say OSX on 3.7?

[dkeeney]

I think it should be sufficient if we manually check 3.7 locally before we check it in. We should only need to do that for a short while.