diff --git a/pre_commit/commands/run.py b/pre_commit/commands/run.py
index 95488b52..d060e186 100644
--- a/pre_commit/commands/run.py
+++ b/pre_commit/commands/run.py
@@ -190,7 +190,7 @@ def _compute_cols(hooks, verbose):
 def _all_filenames(args):
     if args.origin and args.source:
         return git.get_changed_files(args.origin, args.source)
-    elif args.hook_stage in ['prepare-commit-msg', 'commit-msg']:
+    elif args.hook_stage in {'prepare-commit-msg', 'commit-msg'}:
         return (args.commit_msg_filename,)
     elif args.files:
         return args.files
diff --git a/tests/commands/install_uninstall_test.py b/tests/commands/install_uninstall_test.py
index a216bd5a..e253dd4b 100644
--- a/tests/commands/install_uninstall_test.py
+++ b/tests/commands/install_uninstall_test.py
@@ -678,7 +678,7 @@ def test_prepare_commit_msg_integration_passing(
     commit_msg_path = os.path.join(
         prepare_commit_msg_repo, '.git/COMMIT_EDITMSG',
     )
-    with io.open(commit_msg_path, 'rt') as f:
+    with io.open(commit_msg_path) as f:
         assert 'Signed off by: ' in f.read()
 
 
@@ -709,7 +709,7 @@ def test_prepare_commit_msg_legacy(
     commit_msg_path = os.path.join(
         prepare_commit_msg_repo, '.git/COMMIT_EDITMSG',
     )
-    with io.open(commit_msg_path, 'rt') as f:
+    with io.open(commit_msg_path) as f:
         assert 'Signed off by: ' in f.read()
 
 
diff --git a/tests/commands/run_test.py b/tests/commands/run_test.py
index 29534648..b465cae6 100644
--- a/tests/commands/run_test.py
+++ b/tests/commands/run_test.py
@@ -615,7 +615,7 @@ def test_prepare_commit_msg_hook(cap_out, store, prepare_commit_msg_repo):
         stage=False,
     )
 
-    with io.open(filename, 'rt') as f:
+    with io.open(filename) as f:
         assert 'Signed off by: ' in f.read()
 
 
diff --git a/tests/conftest.py b/tests/conftest.py
index e6d7777e..23ff7460 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -171,7 +171,7 @@ def failing_prepare_commit_msg_repo(tempdir_factory):
         'hooks': [{
             'id': 'add-signoff',
             'name': 'Add "Signed off by:"',
-            'entry': '/usr/bin/env bash -c "exit 1"',
+            'entry': 'bash -c "exit 1"',
             'language': 'system',
             'stages': ['prepare-commit-msg'],
         }],