Add pypy to tox by default.
Add pypy to tox by default.

It's quite probable fewer people will have PyPy installed, but I
doubt many will have four versions of CPython installed either.

mgilson/html5lib-python,ordbogen/html5lib-python,dstufft/html5lib-python,mgilson/html5lib-python,gsnedders/html5lib-python,dstufft/html5lib-python,alex/html5lib-python,mgilson/html5lib-python,html5lib/html5lib-python,alex/html5lib-python,html5lib/html5lib-python,gsnedders/html5lib-python,dstufft/html5lib-python,ordbogen/html5lib-python,mindw/html5lib-python,html5lib/html5lib-python,alex/html5lib-python,mindw/html5lib-python,ordbogen/html5lib-python,mindw/html5lib-python