Update ruamel-yaml requirement (#30)

Author: dependabot-preview[bot]

requirements.txt

@@ -1,2 +1,2 @@
-ruamel.yaml >= 0.14.12, <= 0.15.97
+ruamel.yaml >= 0.14.12, < 0.16.6
 typing

setup.py

@@ -25,7 +25,7 @@
     include_package_data=True,
     package_dir={"cwlupgrader.tests": "tests"},
     package_data={"cwlupgrader.tests": ["*.cwl"]},
-    install_requires=["setuptools", "ruamel.yaml >= 0.14.12, <= 0.15.97", "typing"],
+    install_requires=["setuptools", "ruamel.yaml >= 0.14.12, < 0.16.6", "typing"],
     entry_points={"console_scripts": ["cwl-upgrader = cwlupgrader.main:main"]},
     classifiers=[
         "Development Status :: 4 - Beta",