Add support for building universal wheels in the py_wheel rule.

[awbraunstein]

🚀 feature request

Relevant Rules

This is a feature that should be added to the py_wheel rule.

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Description

I'd like the ability to build universal wheels to be added to the py_wheel rule. This should be relatively simple since it just requires passing the --universal flag when building the wheel.

Describe the solution you'd like

Add a new value for python_tag that is either universal or py2and3 that builds a universal wheel.

Describe alternatives you've considered

Building two wheels isn't as easy to distribute.

[pstradomski]

It's probably best to change versions to be a list.

[pstradomski]

Relevant PEPs:

• https://www.python.org/dev/peps/pep-0427/#file-contents
• https://www.python.org/dev/peps/pep-0425/#compressed-tag-sets

[pstradomski]

It would be best to merge https://github.com/bazelbuild/rules_python/pull/388 first, to avoid conflicts