Retire python2 __div__ methods

Suggested by ruff rule PLW3201, which is only part of ruff preview mode, but it seems like a good change to make at this point.

becquerel/core/spectrum.py

@@ -942,7 +942,7 @@ def __mul__(self, other):
     # This line adds the right multiplication
     __rmul__ = __mul__
 
-    def __div__(self, other):
+    def __truediv__(self, other):
         """Return a new Spectrum object with counts (or CPS) scaled down.
 
         Args:
@@ -958,9 +958,6 @@ def __div__(self, other):
 
         return self._mul_div(other, div=True)
 
-    # This line adds true division
-    __truediv__ = __div__
-
     def _mul_div(self, scaling_factor: float, div=False):
         """Multiply or divide a spectrum by a scalar. Handle errors.
 

becquerel/tools/isotope_qty.py

@@ -512,13 +512,8 @@ def __mul__(self, other):
 
         return self._mul_div(other, div=False)
 
-    def __div__(self, other):
-        """Divide the quantity"""
-
-        return self._mul_div(other, div=True)
-
     def __truediv__(self, other):
-        """Divide the quantity (python 3)"""
+        """Divide the quantity"""
 
         return self._mul_div(other, div=True)