test: add regression tests for system pip discovery

tests/test_manager_compat.py

@@ -0,0 +1,55 @@
+from unittest.mock import Mock
+
+from httpie.manager import compat
+
+
+def test_discover_system_pip_falls_back_to_pip3(monkeypatch):
+    # Arrange
+    def which(name):
+        return {
+            "pip": None,
+            "pip3": "/usr/local/bin/pip3",
+        }[name]
+
+    check_output = Mock(
+        return_value="pip 23.0 from /usr/local/lib/python3.12/site-packages/pip (python 3.12)"
+    )
+
+    monkeypatch.setattr(compat.shutil, "which", which)
+    monkeypatch.setattr(compat.subprocess, "check_output", check_output)
+
+    # Act
+    pip = compat._discover_system_pip()
+
+    # Assert
+    assert pip == "/usr/local/bin/pip3"
+    check_output.assert_called_once_with(
+        ["/usr/local/bin/pip3", "--version"], text=True
+    )
+
+
+def test_discover_system_pip_skips_python2_pip(monkeypatch):
+    # Arrange
+    def which(name):
+        return {
+            "pip": "/usr/local/bin/pip",
+            "pip3": "/usr/local/bin/pip3",
+        }[name]
+
+    def fake_check_output(cmd, text=True):
+        if cmd[0] == "/usr/local/bin/pip":
+            return (
+                "pip 20.3 from /usr/local/lib/python2.7/site-packages/pip (python 2.7)"
+            )
+        if cmd[0] == "/usr/local/bin/pip3":
+            return "pip 23.0 from /usr/local/lib/python3.12/site-packages/pip (python 3.12)"
+        raise AssertionError(f"Unexpected executable passed to check_output: {cmd}")
+
+    monkeypatch.setattr(compat.shutil, "which", which)
+    monkeypatch.setattr(compat.subprocess, "check_output", fake_check_output)
+
+    # Act
+    pip = compat._discover_system_pip()
+
+    # Assert
+    assert pip == "/usr/local/bin/pip3"