Specify the Manylinux version to build for with the `MB_ML_VER` environment variable. The default version is `1`. Versions that are currently valid are:
* `1` (see [PEP 513](https://www.python.org/dev/peps/pep-0513);
* `2010` (see PEP
* `2010` (see [PEP
571](https://www.python.org/dev/peps/pep-0571).
At some point `2014` will be a valid version - see [PEP