Add compatibility for Python 3.6: follow-up

This is a follow-up on !132 (merged) which fixes some stuff that I missed.

tests/test_install.py

@@ -67,7 +67,7 @@ def test_install_environment_zlib(create_mock_git_repository):
     #     subprocess.run(
     #         "cp -r foss2021a-mpi fuss1999a",
     #         shell=True,
-    #         stdin=subprocess.PIPE,
+    #         stderr=subprocess.PIPE,
     #         stdout=subprocess.PIPE,
     #     )
     # add zlib as a spec to global_generic
@@ -93,7 +93,7 @@ def test_install_environment_zlib(create_mock_git_repository):
         subprocess.run(
             ["gcc -dumpfullversion"],
             shell=True,
-            stdin=subprocess.PIPE,
+            stderr=subprocess.PIPE,
             stdout=subprocess.PIPE,
         )
         .stdout.decode("utf-8")