Pin PyYAML to <=5.2

This change pins PyYAML, an upstream dependency for bandit, to be
<=5.2. Version 5.3+ drops support for Python 3.4, which breaks any
tests run using Python 3.4. When PyKMIP drops support for Python
3.4, this change can be reverted.

.travis.yml

@@ -231,6 +231,11 @@ install:
   # versions of tox will also drop support for Python 2.7 and 3.4.
   # Once PyKMIP also drops support for Python 2.7 and 3.4, remove this. 
   - pip install "tox<=3.14.1"
+  # Pin PyYAML to <= 5.2. Version 5.3+ removes support for Python 3.4.
+  # PyYAML is installed as a bandit dependency, so while it doesn't
+  # break PyKMIP proper, it does break any of our test runs using
+  # Python 3.4. Once PyKMIP drops support for Python 3.4, remove this.
+  - pip install "PyYAML<=5.2"
   - pip install bandit
   - pip install codecov
   - pip install slugs

test-requirements.txt

@@ -6,4 +6,5 @@ fixtures
 mock
 slugs
 sphinx
+PyYAML<=5.2
 bandit==1.5.1