pytest: Updated requirements.txt to latest versions

This is an issue that was raised in #2665
 
: some of the dependencies where
causing warnings to be added to the logs about deprecated dependencies. Since
I did not get these warnings I just blanket updated all the dependencies in
the hopes of getting the warnings to resolve.

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

tests/requirements.txt

@@ -1,14 +1,12 @@
-flake8==3.6.0
-pytest==4.1.1
-CherryPy==18.1.0
-Flask==1.0.2
-cheroot==6.5.4
-ephemeral-port-reserve==1.1.0
-flaky==3.5.3
-mako==1.0.14
+flake8==3.7.8
+pytest==5.0.1
+Flask==1.1.1
+cheroot==6.5.5
+ephemeral-port-reserve==1.1.1
+flaky==3.6.0
 pytest-benchmark==3.2.2
-pytest-forked==1.0.1
-pytest-xdist==1.26.0
+pytest-forked==1.0.2
+pytest-xdist==1.29.0
 python-bitcoinlib==0.10.1
-tqdm==4.29.1
+tqdm==4.32.2
 pytest-timeout==1.3.3