Add requirements.txt for system tests

Add requirements.txt for system tests

This file lists the required Python packages and versions for running
system tests. The easiest way to obtain them is:

    pip install -r requirements.txt

The minimum dnspython version is 2.7.0 because it supports TSIG parsing
without validation (for tsig/tests_tsig_hypothesis.py) and wire() (for
names/tests_names.py).

The minimum pytest version was bumped to 7.0.0 because it supports the
collection hook API required by pytest 9.

The minimum hypothesis version was set to 4.41.2 as prior versions might
have issues on FIPS systems.