cpu_rec.py (#13): make check on line 534 an equality check

Python 3.8 prints a syntax warning when an identity check is used with a
literal. Using identity comparisons can lead to unexpected behavior.
See: https://adamj.eu/tech/2020/01/21/why-does-python-3-8-syntaxwarning-for-is-literal/

cpu_rec.py

@@ -531,7 +531,7 @@ def __init__(self, training, length=2, modulo=None, FreqVariant='A'):
         self.tbl_size = 0x100**length
         if   length == 2 and modulo is None:
             self.count = self.count_bigrams
-        elif length == 2 and modulo is 4:
+        elif length == 2 and modulo == 4:
             self.count = self.count_bigrams_mod4
         elif length == 3:
             self.count = self.count_trigrams