Bump minimum Python version to 3.7

It's no longer easy to test with older Python versions, so bump the
minimum to 3.7.

README.md

@@ -1,6 +1,6 @@
 # pylibacl
 
-This is a Python 3.4+ extension module allows you to manipulate the
+This is a Python 3.7+ extension module allows you to manipulate the
 POSIX.1e Access Control Lists present in some OS/file-systems
 combinations.
 
@@ -33,7 +33,8 @@ FreeBSD 7 also has quite good support. If any other platform
 implements the POSIX.1e draft, pylibacl can be used. I heard that
 Solaris does, but I can't test it.
 
-- Python 3.4 or newer. Python 2.4+ was supported in the 0.5.x branch.
+- Python 3.7 or newer. Python 2.4+ was supported in the 0.5.x branch,
+  Python 3.4+ in the 0.6 branch.
 - Operating system:
     - Linux, kernel v2.4 or newer, and the libacl library and
       development packages (all modern distributions should have this,

setup.py

@@ -45,7 +45,7 @@
                              libraries=libs,
                              define_macros=macros,
                              )],
-      python_requires = ">=3.4",
+      python_requires = ">=3.7",
       # Note: doesn't work since it's not a package. Sigh.
       package_data = {
           '': ['py.typed', 'posix1e.pyi'],