build wheel for Python 3.9

[ocefpaf]

Avoid compilation when installing with pip on Python 3.9. See #78

[efiring]

I think we should drop 3.6 while adding 3.7, shouldn't we?

[ocefpaf]

I think we should drop 3.6 while adding 3.7, shouldn't we?

Makes sense. We already dropped 3.6 from testing. Although we do build with an old numpy for "forward compatibility."

I'm just waiting for the tests to pass to make the GHA run only on published tags. We'll have to issue a .post0 version to generate the wheel.

[DocOtak]

Would it make sense to follow NEP 29 re python/numpy support?

[ocefpaf]

Would it make sense to follow NEP 29 re python/numpy support?

We kind of do already. numpy 1.18 was dropped this past month but we don't have to rush to 1.19 just yet. Note that the users will get the latest numpy, just the build is with an old one to ensure compatibility.

We can move to 1.19 when we have a new release of gsw, in a way, we are just "catching up" with old releases here by building the wheels.

[efiring]

Looks reasonable, passes the tests.