restructure interfaces __init__ to avoid local variables and namespace pollution

Author: ndgrigorian

mkl_fft/interfaces/__init__.py

@@ -30,18 +30,17 @@
 try:
     import scipy
 
-    _has_scipy = True
+    del scipy
 except ImportError:
-    _has_scipy = False
-
-if _has_scipy:
+    pass
+else:
     try:
         import mkl
 
-        from . import scipy_fft
+        del mkl
     except ImportError:
         pass
     else:
-        __all__.append("scipy_fft")
+        from . import scipy_fft
 
-del _has_scipy
+        __all__.append("scipy_fft")