Merge pull request #154 from creachadair/py36iss153

Require Python ≥ 3.6.

Author: creachadair

setup.py

@@ -336,8 +336,6 @@ def main():
             "Intended Audience :: Developers",
             "License :: OSI Approved :: Apache Software License",
             "Operating System :: POSIX",
-            "Programming Language :: Python :: 3.4",
-            "Programming Language :: Python :: 3.5",
             "Programming Language :: Python :: 3.6",
             "Programming Language :: Python :: 3.7",
             "Topic :: Software Development :: Libraries"