pytest: Add two more dependencies

pytest was an indirect dependency so far, making that one
explicit, and the timeout plugin should allow us to kill a stuck test
before travis kills it, and thus allow us to see where it got stuck.

Signed-off-by: Christian Decker <@cdecker>

tests/requirements.txt

@@ -1,3 +1,4 @@
+pytest==3.8.1
 CherryPy==17.3.0
 Flask==1.0.2
 cheroot==6.5.2
@@ -8,3 +9,4 @@ pytest-forked==0.2
 pytest-xdist==1.22.2
 python-bitcoinlib==0.7.0
 tqdm==4.26.0
+pytest-timeout==1.3.3