Support uwsgi installed but not being used (#836)

The `ImportError` will be raised if `uwsgi` package is not installed. But if installed yet not _in use_, like during a Django's `./manage.py runserver` or on tests, the raised error will be AttributeError over uwsgi.masterpid .

Co-authored-by: Colton Myers <colton.myers@gmail.com>

Author: alanjds

elasticapm/utils/compat.py

@@ -183,7 +183,7 @@ def postfork(f):
             return f
 
 
-except ImportError:
+except (ImportError, AttributeError):
 
     def postfork(f):
         return f