setup.py: Switch minimum Python version to 3.7 (To allow more than 255 arguments in functions).

Author: enjoy-digital

setup.py

@@ -13,7 +13,7 @@
     download_url         = "https://github.com/enjoy-digital/litesdcard",
     test_suite           = "test",
     license              = "BSD",
-    python_requires      = "~=3.6",
+    python_requires      = "~=3.7",
     packages             = find_packages(exclude=("test*", "sim*", "doc*", "examples*")),
     include_package_data = True,
     entry_points         = {