Restrict PyYAML version

Fix avirshup#65

setup.py

@@ -1,9 +1,6 @@
 from setuptools import setup
 import versioneer
 
-with open("requirements.txt", "r") as reqfile:
-    requirements = [x.strip() for x in reqfile if x.strip()]
-
 setup(
     name="DockerMake",
     version=versioneer.get_version(),
@@ -16,5 +13,5 @@
     description="Build manager for docker images",
     url="https://github.com/avirshup/dockermake",
     entry_points={"console_scripts": ["docker-make = dockermake.__main__:main"]},
-    install_requires=["termcolor", "docker>=4", "pyyaml>=5"],
+    install_requires=["termcolor", "docker>=4", "pyyaml>=5,<6"],
 )