Run core tests in Safari. Partially addresses https://github.com/pyodide/pyodide/issues/989

It runs, but there are several test failures, I'll open separate issues about those.
package:
  name: attrs
  version: 21.4.0
  top-level:
    - attr
    - attrs
source:
  sha256: 2d27e3784d7a565d36ab851fe94887c5eccd6a463168875832a1be79c82828b4
  url: https://files.pythonhosted.org/packages/be/be/7abce643bfdf8ca01c48afa2ddf8308c2308b0c3b239a44e57d020afa0ef/attrs-21.4.0-py2.py3-none-any.whl
requirements:
  run:
    - six
about:
  home: https://www.attrs.org/
  PyPI: https://pypi.org/project/attrs
  summary: Classes Without Boilerplate
  license: MIT

package:
  name: attrs
  version: 21.4.0
  top-level:
    - attr
    - attrs
source:
  sha256: 2d27e3784d7a565d36ab851fe94887c5eccd6a463168875832a1be79c82828b4
  url: https://files.pythonhosted.org/packages/be/be/7abce643bfdf8ca01c48afa2ddf8308c2308b0c3b239a44e57d020afa0ef/attrs-21.4.0-py2.py3-none-any.whl
requirements:
  run:
    - six
about:
  home: https://www.attrs.org/
  PyPI: https://pypi.org/project/attrs
  summary: Classes Without Boilerplate
  license: MIT
