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/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 18afbef..d4d348c 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -1061,7 +1061,7 @@ def test_getgroups(self):
         if sys.platform == 'darwin':
             import sysconfig
             dt = sysconfig.get_config_var('MACOSX_DEPLOYMENT_TARGET') or '10.0'
-            if tuple(int(n) for n in dt.split('.')[0:2]) < (10, 6):
+            if tuple(int(n) for n in str(dt).split('.')[0:2]) < (10, 6):
                 raise unittest.SkipTest("getgroups(2) is broken prior to 10.6")
 
         # 'id -G' and 'os.getgroups()' should return the same