Remove python 2.7 and 3.6.

[janiversen]

Remove CI for 2.7 and 3.6.
Update README.rst to reflect we support python >= 3.7

[janiversen]

@dhoomakethu please have a look, I think I did it correctly, but for CI I really would like a second opinion.

[swamper123]

The rest seems to be fine :)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[janiversen]

The failing 3.7 test is outside our control, it seems installing dependencies etc. takes too long.

[swamper123]

It smells suspicious...in other running actions it got at least to the tests and have been cancelled after ~ 8 min.
This time it was cancelled before the tests .... there is sth. fishy.

[swamper123]

I hate such things, where you can't traceback well ... there was a warning in the logs, but this shouldn't be the cause of the "now I want to run, now I cancel"-Action.
From my side this PR is fine and maybe we find the root of all macOS-Pypy-3.7 evil at some time. ^^

[janiversen]

And as you can see, just restarting the job did the trick, there are indeed something fishy with the 3.7 tests.