build: Stop releasing universal wheels

This reverts commit 3c525ab "Release as a universal wheel".

Python 2 support was definitely dropped in early 2021 in commit a3fc64d,
since then it's no longer useful to build universal wheels.

According to the `wheel` documentation:
> If your project contains no C extensions and is expected to work on
> both Python 2 and 3, you will want to tell wheel to produce universal
> wheels

Partly fixes #501

setup.cfg

@@ -1,6 +1,3 @@
-[bdist_wheel]
-universal = 1
-
 [flake8]
 import-order-style = pep8
 application-import-names = yamllint