Use a sentinel type instead of type(None) for type placeholders when SymPy/mpmath are not installed

Author: Technologicat

unpythonic/mathseq.py

@@ -40,14 +40,17 @@
 from .it import take, rev
 from .gmemo import imemoize, gmemoize
 
+class _NoSuchType:
+    pass
+
 # stuff to support float, mpf and SymPy expressions transparently
 #
 from sys import float_info
 from math import log as math_log, copysign, trunc, floor, ceil
 try:
     from mpmath import mpf, almosteq as mpf_almosteq
 except ImportError:
-    mpf = type(None)
+    mpf = type(_NoSuchType())
     mpf_almosteq = None
 
 def _numsign(x):
@@ -69,7 +72,7 @@ def sign(x):
 except ImportError:
     log = math_log
     sign = _numsign
-    _symExpr = type(None)
+    _symExpr = type(_NoSuchType())
 
 def almosteq(a, b, tol=1e-8):
     """Almost-equality that supports several formats.