requirements: Reduce duplication

Simply installing packages will ensure that most of the dependencies in
'setup.py' are installed, meaning 'test-reqs.txt' need only contain
those necessary for testing.

The only notable change is that the 'simplejson' module is dropped from
the requirements list. This included as a dependency for the PyPy
target, but it appears that this is not necessary today (though it may
have been when the target was added in 2011).

This retains 'setup.py test' which, as noted in the tox docs [1], is
sometimes expected for downstream distribution testing. We may wish to
find a way to synchronize requirements between 'test-reqs.txt' and this
section in the future, but that's work for another day.

[1] https://tox.readthedocs.io/en/latest/example/basic.html#integration-with-setup-py-test-command

Signed-off-by: Stephen Finucane <stephen@that.guru>

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