Fix lint, add release notes

Author: ColCarroll

RELEASE-NOTES.md

@@ -50,7 +50,8 @@
 
 - `nuts_kwargs` and `step_kwargs` have been deprecated in favor of using the standard `kwargs` to pass optional step method arguments.
 - `SGFS` and `CSG` have been removed (Fix for [#3353](https://github.com/pymc-devs/pymc3/issues/3353)). They have been moved to [pymc3-experimental](https://github.com/pymc-devs/pymc3-experimental).
--  References to `live_plot` and corresponding notebooks have been removed.
+- References to `live_plot` and corresponding notebooks have been removed.
+- Function `approx_hessian` was removed, due to `numdifftools` becoming incompatible with current `scipy`. The function was already optional, only available to a user who installed `numdifftools` separately, and not hit on any common codepaths. [#3485](https://github.com/pymc-devs/pymc3/pull/3485).
 
 ## PyMC3 3.6 (Dec 21 2018)
 

pymc3/tests/test_distributions.py

@@ -3,8 +3,8 @@
 
 from .helpers import SeededTest, select_by_precision
 from ..vartypes import continuous_types
-from ..model import Model, Point, Potential, Deterministic
-from ..blocking import DictToVarBijection, DictToArrayBijection, ArrayOrdering
+from ..model import Model, Point, Deterministic
+from ..blocking import DictToVarBijection
 from ..distributions import (
     DensityDist, Categorical, Multinomial, VonMises, Dirichlet,
     MvStudentT, MvNormal, MatrixNormal, ZeroInflatedPoisson,