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.

Author: meshy

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)