This PR adds python_requires into setup.py to prevent pip from installing attrs in unsupported python versions.

I know that currently attrs supports a wide range of python versions but I think that it's always good to have some sanity check =)

Pull Request Check List

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• Added tests for changed code.
• New features have been added to our Hypothesis testing strategy.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file tests/typing_example.py.
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