Fix env-var name for parallel tox tests

When running tests in parallel, we need to add a suffix to Django DB names to avoid clashes. This previously used `TOX_PARALLEL_ENV`, but Tox no longer sets this environment variable.

Tox does set an environment variable called `TOX_ENV_NAME`, which works for this purpose, so use that instead.

pytest_django/fixtures.py

@@ -66,7 +66,7 @@
 def django_db_modify_db_settings_tox_suffix() -> None:
     skip_if_no_django()
 
-    tox_environment = os.getenv("TOX_PARALLEL_ENV")
+    tox_environment = os.getenv("TOX_ENV_NAME")
     if tox_environment:
         # Put a suffix like _py27-django21 on tox workers
         _set_suffix_to_test_databases(suffix=tox_environment)

tests/test_db_setup.py

@@ -367,7 +367,7 @@ def test_db_with_tox_suffix(
         monkeypatch: pytest.MonkeyPatch,
     ) -> None:
         "A test to check that Tox DB suffix works when running in parallel."
-        monkeypatch.setenv("TOX_PARALLEL_ENV", "py37-django22")
+        monkeypatch.setenv("TOX_ENV_NAME", "py37-django22")
 
         django_pytester.create_test_module(
             """
@@ -395,7 +395,7 @@ def test_db_with_empty_tox_suffix(
         monkeypatch: pytest.MonkeyPatch,
     ) -> None:
         "A test to check that Tox DB suffix is not used when suffix would be empty."
-        monkeypatch.setenv("TOX_PARALLEL_ENV", "")
+        monkeypatch.setenv("TOX_ENV_NAME", "")
 
         django_pytester.create_test_module(
             """
@@ -434,7 +434,7 @@ def test_db_with_tox_suffix(
     ) -> None:
         "A test to check that both Tox and xdist suffixes work together."
         pytest.importorskip("xdist")
-        monkeypatch.setenv("TOX_PARALLEL_ENV", "py37-django22")
+        monkeypatch.setenv("TOX_ENV_NAME", "py37-django22")
 
         django_pytester.create_test_module(
             """