fix bug in recognize CH pattern

Author: lhqing

ALLCools/utilities.py

@@ -76,7 +76,7 @@ def parse_mc_pattern(pattern: str) -> set:
         except KeyError:
             raise KeyError(f"Base {base} is not in IUPAC table.")
     context_set = {"".join(i) for i in itertools.product(*all_pos_list)}
-    if pattern = 'CH':
+    if pattern == 'CH':
         context_set.add('CH')
     return context_set