Update fastmath.py to use proper default config

Author: seanlaw

stumpy/fastmath.py

@@ -6,7 +6,7 @@
 from . import config
 
 
-@njit(fastmath=config.STUMPY_FASTMATH_TRUE)
+@njit(fastmath=config.STUMPY_FASTMATH_FASTMATH._ADD_ASSOC)
 def _add_assoc(x, y):  # pragma: no cover
     """
     A dummy function to test the fastmath module