chore: Configure setuptools to produce py3-tagged wheels (#178)

Previously, the wheels published to PyPI were being tagged with `py2.py3` implying Python 2 support, despite having `Require-Python: >= 3.7` in the core metadata: https://pypi.org/project/resolvelib/1.1.0/#files.
This patch fixes that, forcing the build tool chain to produce wheels only having the `py3` tag in their filenames.

pyproject.toml

@@ -53,7 +53,7 @@ find = {where = ["src"], namespaces = false}
 version = {attr = "resolvelib.__version__"}
 
 [tool.distutils.bdist_wheel]
-universal = true
+universal = false  # `py3` only tag
 
 [tool.towncrier]
 package = 'resolvelib'