Log Message:
net/py-magic-wormhole: Adjust comments about test deps

Note that py-test is not listed in setup.py, but experimentally
determined to be needed.  (Thus, a bug I'll file upstream.)

Drop comment about pytest_twisted maybe being needed; tests run and
pass without it being installed and there's no trace of it in the make
test output.  (That seems definitive to me; of course, let me know if
I have it wrong.)