Merge pull request #155 from creachadair/minversion

Include a python_requires clause in the setup file.

Author: creachadair

setup.py

@@ -326,6 +326,7 @@ def main():
         packages=find_packages(),
         exclude=["bblfsh/test.py"],
         keywords=["babelfish", "uast"],
+        python_requires='>=3.6',
         install_requires=["grpcio>=1.13.0", "grpcio-tools>=1.13.0",
                           "docker", "protobuf>=3.4.0"],
         package_data={"": ["LICENSE", "README.md"]},