pytest: Update test dependencies to latest version

Signed-off-by: Christian Decker <decker.christian@gmail.com>

tests/requirements.txt

@@ -1,13 +1,13 @@
-flake8==3.5.0
-pytest==3.8.1
-CherryPy==17.3.0
+flake8==3.6.0
+pytest==4.1.1
+CherryPy==18.1.0
 Flask==1.0.2
-cheroot==6.5.2
+cheroot==6.5.4
 ephemeral-port-reserve==1.1.0
-flaky==3.4.0
-pytest-benchmark==3.1.1
-pytest-forked==0.2
-pytest-xdist==1.22.2
-python-bitcoinlib==0.7.0
-tqdm==4.26.0
+flaky==3.5.3
+pytest-benchmark==3.2.2
+pytest-forked==1.0.1
+pytest-xdist==1.26.0
+python-bitcoinlib==0.10.1
+tqdm==4.29.1
 pytest-timeout==1.3.3