bpo-41627: Distinguish 32 and 64-bit user site packages on Windows (GH-22098)

Also fixes the error message returned when sysconfig fails to interpolate a variable correctly.
diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
index bf04ac5..6c87b06 100644
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -53,12 +53,12 @@
         },
     # NOTE: When modifying "purelib" scheme, update site._get_path() too.
     'nt_user': {
-        'stdlib': '{userbase}/Python{py_version_nodot}',
-        'platstdlib': '{userbase}/Python{py_version_nodot}',
-        'purelib': '{userbase}/Python{py_version_nodot}/site-packages',
-        'platlib': '{userbase}/Python{py_version_nodot}/site-packages',
-        'include': '{userbase}/Python{py_version_nodot}/Include',
-        'scripts': '{userbase}/Python{py_version_nodot}/Scripts',
+        'stdlib': '{userbase}/Python{py_version_nodot_plat}',
+        'platstdlib': '{userbase}/Python{py_version_nodot_plat}',
+        'purelib': '{userbase}/Python{py_version_nodot_plat}/site-packages',
+        'platlib': '{userbase}/Python{py_version_nodot_plat}/site-packages',
+        'include': '{userbase}/Python{py_version_nodot_plat}/Include',
+        'scripts': '{userbase}/Python{py_version_nodot_plat}/Scripts',
         'data': '{userbase}',
         },
     'posix_user': {
@@ -149,10 +149,10 @@
 def _subst_vars(s, local_vars):
     try:
         return s.format(**local_vars)
-    except KeyError:
+    except KeyError as var:
         try:
             return s.format(**os.environ)
-        except KeyError as var:
+        except KeyError:
             raise AttributeError('{%s}' % var) from None
 
 def _extend_dict(target_dict, other_dict):
@@ -431,6 +431,7 @@
     vars['EXE'] = '.exe'
     vars['VERSION'] = _PY_VERSION_SHORT_NO_DOT
     vars['BINDIR'] = os.path.dirname(_safe_realpath(sys.executable))
+    vars['TZPATH'] = ''
 
 #
 # public APIs
@@ -543,10 +544,13 @@
         except AttributeError:
             # sys.abiflags may not be defined on all platforms.
             _CONFIG_VARS['abiflags'] = ''
+        try:
+            _CONFIG_VARS['py_version_nodot_plat'] = sys.winver.replace('.', '')
+        except AttributeError:
+            _CONFIG_VARS['py_version_nodot_plat'] = ''
 
         if os.name == 'nt':
             _init_non_posix(_CONFIG_VARS)
-            _CONFIG_VARS['TZPATH'] = ''
         if os.name == 'posix':
             _init_posix(_CONFIG_VARS)
         # For backward compatibility, see issue19555