diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 5dde27e..3e53399 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9]
 
     steps:
     - name: Check out source code
diff --git a/miplearn/components/component.py b/miplearn/components/component.py
index 67170b2..095d19a 100644
--- a/miplearn/components/component.py
+++ b/miplearn/components/component.py
@@ -2,8 +2,6 @@
 #  Copyright (C) 2020, UChicago Argonne, LLC. All rights reserved.
 #  Released under the modified BSD license. See COPYING.md for more details.
 
-from __future__ import annotations
-
 from abc import ABC, abstractmethod
 from typing import Any, List, Union, TYPE_CHECKING
 
@@ -25,7 +23,7 @@ class Component(ABC):
 
     def before_solve(
         self,
-        solver: LearningSolver,
+        solver: "LearningSolver",
         instance: Instance,
         model: Any,
     ) -> None:
@@ -46,7 +44,7 @@ class Component(ABC):
     @abstractmethod
     def after_solve(
         self,
-        solver: LearningSolver,
+        solver: "LearningSolver",
         instance: Instance,
         model: Any,
         stats: MIPSolveStats,
@@ -57,7 +55,7 @@ class Component(ABC):
 
         Parameters
         ----------
-        solver: LearningSolver
+        solver: "LearningSolver"
             The solver calling this method.
         instance: Instance
             The instance being solved.
@@ -86,7 +84,7 @@ class Component(ABC):
 
     def iteration_cb(
         self,
-        solver: LearningSolver,
+        solver: "LearningSolver",
         instance: Instance,
         model: Any,
     ) -> bool:
@@ -103,7 +101,7 @@ class Component(ABC):
 
         Parameters
         ----------
-        solver: LearningSolver
+        solver: "LearningSolver"
             The solver calling this method.
         instance: Instance
             The instance being solved.
@@ -114,7 +112,7 @@ class Component(ABC):
 
     def lazy_cb(
         self,
-        solver: LearningSolver,
+        solver: "LearningSolver",
         instance: Instance,
         model: Any,
     ) -> None:
diff --git a/miplearn/instance.py b/miplearn/instance.py
index c3d3824..c47f993 100644
--- a/miplearn/instance.py
+++ b/miplearn/instance.py
@@ -10,7 +10,6 @@ from typing import Any, List
 import numpy as np
 
 from miplearn.types import TrainingSample
-import pyomo.environ as pe
 
 
 class Instance(ABC):
diff --git a/miplearn/problems/tests/test_knapsack.py b/miplearn/problems/tests/test_knapsack.py
index 61f0402..553f895 100644
--- a/miplearn/problems/tests/test_knapsack.py
+++ b/miplearn/problems/tests/test_knapsack.py
@@ -19,8 +19,6 @@ def test_knapsack_generator():
     )
     instances = gen.generate(100)
     w_sum = sum(instance.weights for instance in instances) / len(instances)
-    p_sum = sum(instance.prices for instance in instances) / len(instances)
     b_sum = sum(instance.capacities for instance in instances) / len(instances)
     assert round(np.mean(w_sum), -1) == 500.0
-    # assert round(np.mean(p_sum), -1) == 1200.  # flaky
     assert round(np.mean(b_sum), -3) == 25000.0
diff --git a/miplearn/solvers/learning.py b/miplearn/solvers/learning.py
index 8c4d464..5a540f6 100644
--- a/miplearn/solvers/learning.py
+++ b/miplearn/solvers/learning.py
@@ -7,7 +7,6 @@ import logging
 import os
 import pickle
 import tempfile
-from copy import deepcopy
 from typing import Optional, List, Any, IO, cast, BinaryIO, Union, Callable, Dict
 
 from p_tqdm import p_map
diff --git a/miplearn/solvers/tests/test_internal_solver.py b/miplearn/solvers/tests/test_internal_solver.py
index e05a23a..b7a0a24 100644
--- a/miplearn/solvers/tests/test_internal_solver.py
+++ b/miplearn/solvers/tests/test_internal_solver.py
@@ -7,7 +7,6 @@ from io import StringIO
 from warnings import warn
 
 import pyomo.environ as pe
-from pytest import raises
 
 from miplearn.solvers import RedirectOutput
 from miplearn.solvers.gurobi import GurobiSolver