python ver dependency

lowered python version requires to 3.6 #186

Author: jchanvfx

setup.py

@@ -17,7 +17,7 @@
 classifiers = [
     'Operating System :: OS Independent',
     'License :: OSI Approved :: MIT License',
-    'Programming Language :: Python :: 3.7',
+    'Programming Language :: Python :: 3.6',
     'Documentation :: https://jchanvfx.github.io/NodeGraphQt/api/html/index.html',
     'Source :: https://github.com/jchanvfx/NodeGraphQt/',
 ]
@@ -35,7 +35,7 @@
     classifiers=classifiers,
     install_requires=requirements,
     include_package_data=True,
-    python_requires='>=3.7'
+    python_requires='>=3.6'
 )