tox: Use compatible minor releases

tox: Use compatible minor releases

As we previously did for the requirements files (commit c90473ea44),
switch to use compatible releases for tox. We previously avoided doing
this on the basis that the existing system gave an idea of supported
package versions, but we weren't actually testing these combinations
since pip will always install the latest available version of each
package. This will apply to anyone using pip to manage their
dependencies, while anyone using distro packages can rely on the distro
having done this testing for them. Given the above, this makes our
effort to track supported ranges moot. So long as we cap newer versions
of e.g. django-filter when they're not compatible with older Django
versions, we should be good.

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