[PYLINT FIX]pylint issue fixes (#348)

Before Fix
************* 
************* Module nnvm._base
R: 21, 0: Disallow trailing comma tuple (trailing-comma-tuple)
R: 27, 0: Disallow trailing comma tuple (trailing-comma-tuple)

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Your code has been rated at 9.99/

After Fix
************* 
siju@siju-DAEMON:~/D/Community/nnvm$ make lint
pylint python/nnvm --rcfile=/media/siju/DATA/Community/nnvm/tests/lint/pylintrc

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Your code has been rated at 10.00/10 (previous run: 9.99/10, +0.01)

nnvm/python/nnvm/_base.py

@@ -18,13 +18,13 @@ except ImportError:
 # library loading
 #----------------------------
 if sys.version_info[0] == 3:
-    string_types = str,
+    string_types = str
     numeric_types = (float, int, np.float32, np.int32)
     # this function is needed for python3
     # to convert ctypes.char_p .value back to python str
     py_str = lambda x: x.decode('utf-8')
 else:
-    string_types = basestring,
+    string_types = basestring
     numeric_types = (float, int, long, np.float32, np.int32)
     py_str = lambda x: x