Since pytest itself skips tests using dnspython if the latter is not
available, also using Automake conditionals for silently skipping
pytest-based tests requiring dnspython is redundant and hides
information. Allow all pytest-based tests requiring dnspython to be run
whenever pytest itself is available, in order to ensure test skipping is
done in a uniform manner.
Note that the above reasoning only applies to pytest-based tests, so
similar adjustments were not made for shell-based tests using Python
scripts that require dnspython ("chain", "cookie", "dnssec", "qmin").
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