[3.8] bpo-39794: Add --without-decimal-contextvar (GH-18702)

(cherry picked from commit 815280eb160af637e1347213659f9236adf78f80)
diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
index c14d8ca..ab989e5 100644
--- a/Lib/_pydecimal.py
+++ b/Lib/_pydecimal.py
@@ -140,8 +140,11 @@
     # Limits for the C version for compatibility
     'MAX_PREC',  'MAX_EMAX', 'MIN_EMIN', 'MIN_ETINY',
 
-    # C version: compile time choice that enables the thread local context
-    'HAVE_THREADS'
+    # C version: compile time choice that enables the thread local context (deprecated, now always true)
+    'HAVE_THREADS',
+
+    # C version: compile time choice that enables the coroutine local context
+    'HAVE_CONTEXTVAR'
 ]
 
 __xname__ = __name__    # sys.modules lookup (--without-threads)
@@ -172,6 +175,7 @@
 
 # Compatibility with the C version
 HAVE_THREADS = True
+HAVE_CONTEXTVAR = True
 if sys.maxsize == 2**63-1:
     MAX_PREC = 999999999999999999
     MAX_EMAX = 999999999999999999