No OpenMPI build variant

- [x] I read [the conda-forge documentation](https://conda-forge.org/docs/user/introduction.html#how-can-i-install-packages-from-conda-forge) and could not find the solution for my problem there.

Issue:

There is no OpenMPI build variant for this package (only mpich or nompi).
This is not a high priority issue, but may be important for users who wish to install Kwant alongside software that only works with OpenMPI.

We had to disable building the OpenMPI variant, as tests failed in a strange way when building the package:


Environment (conda list):

```
$ conda list

```

Details about conda and system ( conda info ):

```
$ conda info

```