Updated python version requirement to 3.7 (#3877)

With `pymdown-extensions` required version recently changed to 9.4, `mkdocs-material` effectively requires at least python 3.7 now

Author: Dennis Pan

setup.py

@@ -64,7 +64,7 @@
     packages = find_packages(exclude = ["src", "src.*"]),
     include_package_data = True,
     install_requires = install_requires,
-    python_requires='>=3.6',
+    python_requires='>=3.7',
     entry_points = {
         "mkdocs.themes": [
             "material = material"