Fix typing for histogram scalar division

The division was already supported - the typing just needed to be
updated (following the example of scalar multiplication).

pachyderm/histogram.py

@@ -500,20 +500,23 @@ def __imul__(self: _T, other: Union[_T, float]) -> _T:
             self.y *= other.y
         return self
 
-    def __truediv__(self: _T, other: _T) -> _T:
+    def __truediv__(self: _T, other: Union[_T, float]) -> _T:
         """ Handles ``a = b / c``. """
         new = self.copy()
         new /= other
         return new
 
-    def __itruediv__(self: _T, other: _T) -> _T:
+    def __itruediv__(self: _T, other: Union[_T, float]) -> _T:
         """ Handles ``a /= b``. """
         if np.isscalar(other):
             # Help out mypy...
             assert isinstance(other, (float, int, np.number))
             # Scale histogram by a scalar
             self *= 1. / other
         else:
+            # Help out mypy...
+            assert isinstance(other, Histogram1D)
+            # Validation
             if not np.allclose(self.bin_edges, other.bin_edges):
                 raise TypeError(
                     f"Binning is different for given histograms."