bpo-42504: fix for MACOSX_DEPLOYMENT_TARGET=11 (GH-23556)

macOS releases numbering has changed as of macOS 11 Big Sur.  Previously, major releases were of the form 10.x, 10.x+1, 10.x+2, etc; as of Big Sur, they are now x, x+1, etc, so, for example, 10.15, 10.15.1, ..., 10.15.7, 11, 11.0.1, 11.1, ..., 12, 12.1, etc. Allow Python to build with single-digit deployment target values. Patch provided by FX Coudert.
diff --git a/setup.py b/setup.py
index b7a7d26..0c9a425 100644
--- a/setup.py
+++ b/setup.py
@@ -1014,7 +1014,7 @@
             os_release = int(os.uname()[2].split('.')[0])
             dep_target = sysconfig.get_config_var('MACOSX_DEPLOYMENT_TARGET')
             if (dep_target and
-                    (tuple(int(n) for n in dep_target.split('.')[0:2])
+                    (tuple(int(n) for n in str(dep_target).split('.')[0:2])
                         < (10, 5) ) ):
                 os_release = 8
             if os_release < 9: