Fix crash on Python 2 when reusing environments (#450)

* Remove redundant mock in test for environment reuse

This breaks tests comparing a string from pyvenv.cfg to command output, because
the latter will be a mock. There is no inherent need for this mock. It was added
recently, and probably speculatively or for performance reasons.

* Add test case for reusing Python 2 environments

* Read base prefix from pyvenv.cfg if present

* Remove trailing newline when querying interpreter base prefix

nox/virtualenv.py

@@ -339,15 +339,35 @@ def _check_reused_environment_type(self) -> bool:
 
     def _check_reused_environment_interpreter(self) -> bool:
         """Check if reused environment interpreter is the same."""
-        program = "import sys; print(getattr(sys, 'real_prefix', sys.base_prefix))"
-        original = nox.command.run(
-            [self._resolved_interpreter, "-c", program], silent=True, log=False
+        original = self._read_base_prefix_from_pyvenv_cfg()
+        program = (
+            "import sys; sys.stdout.write(getattr(sys, 'real_prefix', sys.base_prefix))"
         )
+
+        if original is None:
+            output = nox.command.run(
+                [self._resolved_interpreter, "-c", program], silent=True, log=False
+            )
+            assert isinstance(output, str)
+            original = output
+
         created = nox.command.run(
             ["python", "-c", program], silent=True, log=False, paths=self.bin_paths
         )
+
         return original == created
 
+    def _read_base_prefix_from_pyvenv_cfg(self) -> Optional[str]:
+        """Return the base-prefix entry from pyvenv.cfg, if present."""
+        path = os.path.join(self.location, "pyvenv.cfg")
+        if os.path.isfile(path):
+            with open(path) as io:
+                for line in io:
+                    key, _, value = line.partition("=")
+                    if key.strip() == "base-prefix":
+                        return value.strip()
+        return None
+
     @property
     def _resolved_interpreter(self) -> str:
         """Return the interpreter, appropriately resolved for the platform.

tests/test_virtualenv.py

@@ -335,7 +335,6 @@ def test_create(monkeypatch, make_one):
     dir_.ensure("test.txt")
     assert dir_.join("test.txt").check()
     venv.reuse_existing = True
-    monkeypatch.setattr(nox.virtualenv.nox.command, "run", mock.MagicMock())
 
     venv.create()
 
@@ -443,6 +442,19 @@ def test_create_reuse_oldstyle_virtualenv_environment(make_one):
     assert reused
 
 
+def test_create_reuse_python2_environment(make_one):
+    venv, location = make_one(reuse_existing=True, interpreter="2.7")
+
+    try:
+        venv.create()
+    except nox.virtualenv.InterpreterNotFound:
+        pytest.skip("Requires Python 2.7 installation.")
+
+    reused = not venv.create()
+
+    assert reused
+
+
 def test_create_venv_backend(make_one):
     venv, dir_ = make_one(venv=True)
     venv.create()