Address copilot review comments

Assisted-by: Codex.app:GPT-5.4

Author: dwhswenson

.github/workflows/unit-tests.yml

@@ -44,7 +44,7 @@ jobs:
             INSTALL_WITH: "conda"
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 2
       - uses: conda-incubator/setup-miniconda@v2

contact_map/tests/test_contact_count.py

@@ -1,12 +1,5 @@
 import numpy as np
 
-try:
-    import matplotlib
-except ImportError:
-    pass
-else:
-    matplotlib.use("Agg")
-
 # pylint: disable=wildcard-import, missing-docstring, protected-access
 # pylint: disable=attribute-defined-outside-init, invalid-name, no-self-use
 # pylint: disable=wrong-import-order, unused-wildcard-import