Fix runtime loading error message

Author: filmor

pythonnet/__init__.py

@@ -1,4 +1,3 @@
-import os
 import sys
 import clr_loader
 
@@ -11,7 +10,7 @@
 def set_runtime(runtime):
     global _RUNTIME
     if _LOADED:
-        raise RuntimeError("The runtime {runtime} has already been loaded".format(_RUNTIME))
+        raise RuntimeError("The runtime {} has already been loaded".format(_RUNTIME))
 
     _RUNTIME = runtime