TST: More isolation for required dep test

Setting PYTHONPATH to include a dir with these libs breaks
the test. We use -E to disable that.

Closes pandas-dev#26983

Author: TomAugspurger

pandas/tests/test_downstream.py

@@ -135,8 +135,11 @@ def test_pyarrow(df):
 
 def test_missing_required_dependency():
     # GH 23868
-    # use the -S flag to disable site-packages
-    call = ['python', '-S', '-c', 'import pandas']
+    # To ensure proper isolation, we pass these flags
+    # -S : disable site-packages
+    # -s : disable user site-packages
+    # -E : disable PYTHON* env vars, especially PYTHONPATH
+    call = ['python', '-S', '-s', '-E', '-c', 'import pandas']
 
     with pytest.raises(subprocess.CalledProcessError) as exc:
         subprocess.check_output(call, stderr=subprocess.STDOUT)