Use a gensym to represent failed imports where possible

Technologicat · Technologicat · commit 511dd2ff9564 · 2020-08-09T02:38:12.000+03:00
unpythonic.syntax.astcompat already used this, so this is just to make the other
modules consistent with it.
diff --git a/unpythonic/mathseq.py b/unpythonic/mathseq.py
@@ -50,6 +50,7 @@ class _NoSuchType:
 try:
     from mpmath import mpf, almosteq as mpf_almosteq
 except ImportError:
+    # Can't use a gensym here since `mpf` must be a unique *type*.
     mpf = type(_NoSuchType())
     mpf_almosteq = None
 
diff --git a/unpythonic/syntax/astcompat.py b/unpythonic/syntax/astcompat.py
@@ -1,7 +1,9 @@
 # -*- coding: utf-8 -*-
 """Conditionally import AST node types only supported by Python 3.5+ or 3.6+."""
 
-_NoSuchNodeType = object()
+from ..symbol import gensym
+
+_NoSuchNodeType = gensym("_NoSuchNodeType")
 
 try:  # Python 3.5+
     from ast import AsyncFor, AsyncFunctionDef, AsyncWith, Await, MatMult
