Adds a dependency for pytest and configurates it to search for files following the pytest_* scheme.

After the transition to pytest is completed and the nose-based testsuite can be safely removed, it is probably a good idea to rename the pytest-based testsuite back to the current test_* scheme.

While having both testsuites at the same time causes temporary code duplication, it makes the transition process easier and allows quick three-way diffs between the main branch and the, on this branch, unmodified tests (via git diff) and the changes specific to pytest (via regular diff on this branch).

The same already applies to the existing nose-based testsuite - using wildcard imports shortens test code significantly

Saves the coverage of both testsuites, to allow us to compare them (and make sure the nose-based testsuite hasn't had a regression for some reason)

…assignment, test_pqos to pytest

… from default context

The current implementation fails under pytest, as the context (as given by getcontext())
is already derived from the DefaultContext *before* we change the rounding to our
preferred value. Thus, the assertion correctly fails, as the first process still has
the default ROUND_HALF_EVEN value, even though all decending processes do possess the
correct value of ROUND_HALF_UP.

As a solution, we can copy the DefaultContext as the context of the initial thread.
As the assertion afterwards becomes very obviously true, additional assertions are added,
to check if the individual processes have the correct rounding.

… context

now requires no assertion to be violated when the rounding mode is modified for the thread,
which should always be the case as long as local context is used (which sets the rounding mode
independently of the rounding mode of the actual, global decimal context of the thread

… extend localcontext to additonal methods

…mode, formerly set for the whole thread

Since "setup.py test" is no longer possible,
we need to tell people.

This fastens up CI jobs by not having to install it on every job.