chore(deps): nuke decorator dependency (#651)

Either this is unused and we can avoid it as unnecessary or it's
somehow used by magic and I'm about to learn something.

Author: TheKevJames

setup.py

@@ -30,11 +30,6 @@ def get_readme_md_contents():
     'configparser<5;python_version<"3.0"',
 ]
 
-if sys.version_info < (3, 0):
-    install_reqs.insert(0, "decorator<5.0.0")
-else:
-    install_reqs.append("decorator>=3.3.2")
-
 setup(
     name="datadog",
     version=version["__version__"],