Drop support for Python 2.7 and 3.5

[Pike]

... and update our testing and compat matrix beyond 3.5, too.

This should be done syntax first, and then the other two.

[Pike]

As we're breaking compat, should we also move from pgkutil-style namespace packages to native ones?

This would be a good time, as we could just generally bump mutual requirements on all three packages. Also for fluent.migration, I guess.

https://packaging.python.org/guides/packaging-namespace-packages/#native-namespace-packages

[sandalwoodbox]

Was this resolved by #161 & #163?

[eemeli]

As noted above, support for Python < 3.6 has already been dropped.