Fix python_requires to support all versions of 3.7

This is the standard way to support "Python 3.7 and newer" and it also
fixes use of borgmatic with some tools that do custom dependency
resolution.  E.g., using pex with --platform.

setup.py

@@ -37,5 +37,5 @@ setup(
         'colorama>=0.4.1,<0.5',
     ),
     include_package_data=True,
-    python_requires='>3.7.0',
+    python_requires='>=3.7',
 )