set minimum python version to 3.5

@GregAshton as discussed last year with @ReinhardPrix in #20 (closed), I'd like to

1. set the minimum version to 3.5 right now (which is what I still have on Debian Stretch)
2. make a Zenodo release (will need you to do this)
3. bump again to 3.6 (minimum required by flake8).

setup.py

@@ -2,6 +2,19 @@
 
 from setuptools import setup, find_packages
 from os import path
+import sys
+
+# check python version
+min_python_version = (3, 5, 0)  # (major,minor,micro)
+python_version = sys.version_info
+print("Running Python version %s.%s.%s" % python_version[:3])
+if python_version < min_python_version:
+    sys.exit(
+        "Python < %s.%s.%s is not supported, aborting setup" % min_python_version[:3]
+    )
+else:
+    print("Confirmed Python version %s.%s.%s or above" % min_python_version[:3])
+
 
 here = path.abspath(path.dirname(__file__))
 # Get the long description from the README file
@@ -11,8 +24,9 @@ with open(path.join(here, "README.md"), encoding="utf-8") as f:
 setup(
     name="PyFstat",
     version="0.2",
-    author="Gregory Ashton",
+    author="Gregory Ashton, David Keitel, Reinhard Prix",
     author_email="gregory.ashton@ligo.org",
+    license="MIT",
     description="python wrappers for lalpulsar F-statistic code",
     long_description=long_description,
     long_description_content_type="text/markdown",
@@ -24,6 +38,7 @@ setup(
             "pyCUDAkernels/cudaTransientFstatRectWindow.cu",
         ]
     },
+    python_requires=">=%s.%s.%s" % min_python_version[:3],
     install_requires=[
         "matplotlib",
         "scipy",