try to find pytest-cov for older python versions

pyproject.toml

@@ -23,6 +23,9 @@ classifiers=[
 [project.optional-dependencies]
 testing = [
     "pytest>=8.0",
-    "pytest-cov>=5,<6",  # Use pytest-cov version 5.x which supports Python 3.8
-    "requests>=2.0"
+    "requests>=2.0",
+    # For Python 3.9 and up, use pytest-cov>=6.0
+    'pytest-cov>=6.0; python_version >= "3.9"',
+    # For older versions, use an older version of pytest-cov or omit it:
+    'pytest-cov>=5,<6; python_version < "3.9"'
 ]
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