update setup.py: only python 3.7 is supported.

As a result of using from ```__future__ import annotations```, the bunq SDK is no longer compatible with Python versions lower than 3.7.

Author: csdenboer

setup.py

@@ -58,8 +58,6 @@
 
         # Specify the Python versions you support here. In particular, ensure
         # that you indicate whether you support Python 2, Python 3 or both.
-        'Programming Language :: Python :: 3.5',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
     ],