WIP: Re-enabling tests for Python3.4

[funkyfuture]

this is my first approach to re-enable tests for Python 3.4. i'm sparse with time, so i would appreciate if anyone could investigate this issue further or come up with other proposals. here are some noteworthy facts:

• the Cerberus 1.3 line aims to support all releases that its initial release and the following did
• the last pytest release for Python 3.4 was 4.6
• the pytest 4.6.11 release declares attrs>=17.40 as dependency
• the last attrs release that supported Python 3.4 was 20.3.0
• but pip resolves the dependency to 21.1.0 (though it has been yanked and a newer release is available)

hence i tried to replicate what is possible on the command line: pip install pytest && pip install attrs==20.3.0, but tox / pip don't seem to consider the version constraint.