Stop install tox within the tox environment (#150)
Stop install tox within the tox environment (#150)

After tox is executed, it is not necessary to install tox in the newly
created virtualenv. Simplify the tox dependencies to just the single one
required, coverage.

This makes the requirements-dev.txt unnecessary and unused, so can
remove it. Its pinned versions are outdated anyway.
pyparsing/pyparsing,pyparsing/pyparsing