Add Python 3.7+ version requirement

Fixes #124

Author: aleksihakli

setup.py

@@ -12,7 +12,7 @@
     long_description=open("README.rst").read() + "\n\n" + open("CHANGES.rst").read(),
     long_description_content_type="text/x-rst",
     license="MIT license",
-    python_requires=">=3.6",
+    python_requires=">=3.7",
     requires=["django (>=2.2)"],
     packages=["widget_tweaks", "widget_tweaks.templatetags"],
     classifiers=[