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/distutils/spawn.py b/Lib/distutils/spawn.py
index 0d1bd03..f50edd2 100644
--- a/Lib/distutils/spawn.py
+++ b/Lib/distutils/spawn.py
@@ -54,8 +54,8 @@ def spawn(cmd, search_path=1, verbose=0, dry_run=0):
         global _cfg_target, _cfg_target_split
         if _cfg_target is None:
             from distutils import sysconfig
-            _cfg_target = sysconfig.get_config_var(
-                                  'MACOSX_DEPLOYMENT_TARGET') or ''
+            _cfg_target = str(sysconfig.get_config_var(
+                                  'MACOSX_DEPLOYMENT_TARGET') or '')
             if _cfg_target:
                 _cfg_target_split = [int(x) for x in _cfg_target.split('.')]
         if _cfg_target: