Optimized @async_unsafe.

Switched the order of the checks to reduce the overhead. Async unsafe methods
are *normally* called syncrhonously, so we can avoid the overhead of checking
the environment variable in the regular path.

django/utils/asyncio.py

@@ -1,6 +1,6 @@
-import asyncio
-import functools
 import os
+from asyncio import get_running_loop
+from functools import wraps
 
 from django.core.exceptions import SynchronousOnlyOperation
 
@@ -11,15 +11,15 @@ def async_unsafe(message):
     the function while in an async context will get an error message.
     """
     def decorator(func):
-        @functools.wraps(func)
+        @wraps(func)
         def inner(*args, **kwargs):
-            if not os.environ.get('DJANGO_ALLOW_ASYNC_UNSAFE'):
             # Detect a running event loop in this thread.
             try:
-                    asyncio.get_running_loop()
+                get_running_loop()
             except RuntimeError:
                 pass
             else:
+                if not os.environ.get('DJANGO_ALLOW_ASYNC_UNSAFE'):
                     raise SynchronousOnlyOperation(message)
             # Pass onward.
             return func(*args, **kwargs)