Reduce python requirement from 3.6 to 3.5.

Currently, python3 in debian releases is version 3.5 and version 3.6
is only available in testing (buster).

In addition, there should not be anything in the code that specially
requires python 3.6.

setup.py

@@ -50,6 +50,6 @@
     ],
     packages=find_packages(),
     package_data={'bootstrapform_jinja': template_files},
-    python_requires='>=3.6',
+    python_requires='>=3.5',
     zip_safe=True,
 )