Building OpenMM for Python 3.8

[jchodera]

We're going to need to build OpenMM for Python 3.8 now that conda defaults to installing this when creating new environments.

I'm hoping we can tackle this as part of the conda-forge migration, since I'm not sure how we would add Python 3.8 to our current build-release procedure since our current scheme would have us test it for a while before releasing.

[peastman]

If this looks like it's going to be a major ordeal to get working, I think it's ok for OpenMM 7.4.1 to only have packages for the same Python versions as 7.4. Supporting a new Python version is the sort of thing it's reasonable to say only happens in major updates, not patches.

[jchodera]

That's not going to work here. We (the users and developers of downstream packages) need Python 3.8 support for some OpenMM package ASAP, be in 7.4.0 or 7.4.1.

We need a plan now.

[jchodera]

Notably, @jaimergp has already got this working for the conda-forge test builds, so that's ready to go: conda-forge/openmm-feedstock#13

[peastman]

As you choose. In any case, we don't need to hold up 7.4.1 for this. We can go ahead and release packages for all the same versions as 7.4, then add Python 3.8 once that's ready.

[jchodera]

We're trying to extend the omnia infrastructure to build for python 3.8, but it looks like this could require major development effort:

omnia-md/conda-recipes#999