Scheduled monthly dependency update for April

requirements.txt

@@ -1 +1 @@
-PyYAML==5.1.2
+PyYAML==5.3.1

requirements_dev.txt

@@ -1,11 +1,11 @@
 bumpversion==0.5.3
-wheel==0.33.6
-watchdog==0.9.0
-flake8==3.7.8
-tox==3.13.2
-Sphinx==2.2.0
-coverage==4.5.4
-cryptography==2.7
-PyYAML==5.1.2
-pytest==5.1.2
-pytest-runner==5.1
+wheel==0.34.2
+watchdog==0.10.2
+flake8==3.7.9
+tox==3.14.6
+Sphinx==2.4.4
+coverage==5.0.4
+cryptography==2.8
+PyYAML==5.3.1
+pytest==5.4.1
+pytest-runner==5.2