Don't build universal wheels for Python 3-only

[hugovk]

Universal wheels are for when you need to support both Python 2 and 3:

https://packaging.python.org/guides/distributing-packages-using-setuptools/#wheels

Before:

• jsonschema-0.1.dev1+gdbda4f7-py2.py3-none-any.whl

After:

• jsonschema-0.1.dev2+g97057ed-py3-none-any.whl

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Also ignore some build dirs generated by running python -m build locally.

[codecov]

Codecov Report

Merging #841 (3da1fcf) into main (dbda4f7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #841   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          19       19           
  Lines        3124     3124           
  Branches      378      378           
=======================================
  Hits         3071     3071           
  Misses         41       41           
  Partials       12       12           

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[Julian]

Thanks!