以下のコードを実行すると

AttributeError: module 'tensorflow.python.keras.backend' has no attribute 'get_graph'

と表示されます。何故でしょうか？
Tensorflowのバージョンは1.15.0
Kerasのバージョンは2.3.1です。

import keras
from keras.models import Sequential
from keras.layers import Dense, Dropout, Flatten
from keras.layers import Conv2D, MaxPooling2D
from keras.optimizers import RMSprop 
from keras.datasets import mnist
import matplotlib.pyplot as plt

# 入力と出力を指定 --- (*1)
im_rows = 28 # 画像の縦ピクセルサイズ
im_cols = 28 # 画像の横ピクセルサイズ
im_color = 1 # 画像の色空間/グレイスケール
in_shape = (im_rows, im_cols, im_color)
out_size = 10

# MNISTのデータを読み込み 
(X_train, y_train), (X_test, y_test) = mnist.load_data()
# 読み込んだデータをの三次元配列に変換 --- (*1a)
X_train = X_train.reshape(-1, im_rows, im_cols, im_color)
X_train = X_train.astype('float32') / 255
X_test = X_test.reshape(-1, im_rows, im_cols, im_color)
X_test = X_test.astype('float32') / 255
# ラベルデータをone-hotベクトルに直す
y_train = keras.utils.np_utils.to_categorical(y_train.astype('int32'),10)
y_test = keras.utils.np_utils.to_categorical(y_test.astype('int32'),10)

# CNNモデル構造を定義 --- (*2)
model = Sequential()
model.add(Conv2D(32, 
          kernel_size=(3, 3),
          activation='relu',
          input_shape=in_shape))
model.add(Conv2D(64, (3, 3), activation='relu'))
model.add(MaxPooling2D(pool_size=(2, 2)))
model.add(Dropout(0.25))
model.add(Flatten())
model.add(Dense(128, activation='relu'))
model.add(Dropout(0.5))
model.add(Dense(out_size, activation='softmax'))

# モデルをコンパイル --- (*3)
model.compile(
    loss='categorical_crossentropy',
    optimizer=RMSprop(),
    metrics=['accuracy'])

# 学習を実行 --- (*4)
hist = model.fit(X_train, y_train,
          batch_size=128, 
          epochs=12,
          verbose=1,
          validation_data=(X_test, y_test))

# モデルを評価 --- (*5)
score = model.evaluate(X_test, y_test, verbose=1)
print('正解率=', score[1], 'loss=', score[0])

# 学習の様子をグラフへ描画 --- (*6)
# 正解率の推移をプロット
plt.plot(hist.history['acc'])
plt.plot(hist.history['val_acc'])
plt.title('Accuracy')
plt.legend(['train', 'test'], loc='upper left')
plt.show()

# ロスの推移をプロット
plt.plot(hist.history['loss'])
plt.plot(hist.history['val_loss'])
plt.title('Loss')
plt.legend(['train', 'test'], loc='upper left')
plt.show()

---------------------------------------------
以下エラー内容
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-1-20848a701fc1> in <module>
     27 
     28 # CNNモデル構造を定義 --- (*2)
---> 29 model = Sequential()
     30 model.add(Conv2D(32, 
     31           kernel_size=(3, 3),

~/opt/anaconda3/envs/python/lib/python3.7/site-packages/keras/engine/sequential.py in __init__(self, layers, name)
     85 
     86     def __init__(self, layers=None, name=None):
---> 87         super(Sequential, self).__init__(name=name)
     88         self._build_input_shape = None
     89 

~/opt/anaconda3/envs/python/lib/python3.7/site-packages/keras/legacy/interfaces.py in wrapper(*args, **kwargs)
     89                 warnings.warn('Update your `' + object_name + '` call to the ' +
     90                               'Keras 2 API: ' + signature, stacklevel=2)
---> 91             return func(*args, **kwargs)
     92         wrapper._original_function = func
     93         return wrapper

~/opt/anaconda3/envs/python/lib/python3.7/site-packages/keras/engine/network.py in __init__(self, *args, **kwargs)
     94         else:
     95             # Subclassed network
---> 96             self._init_subclassed_network(**kwargs)
     97 
     98     def _base_init(self, name=None):

~/opt/anaconda3/envs/python/lib/python3.7/site-packages/keras/engine/network.py in _init_subclassed_network(self, name)
    292 
    293     def _init_subclassed_network(self, name=None):
--> 294         self._base_init(name=name)
    295         self._is_graph_network = False
    296         self._expects_training_arg = has_arg(self.call, 'training')

~/opt/anaconda3/envs/python/lib/python3.7/site-packages/keras/engine/network.py in _base_init(self, name)
    107         if not name:
    108             prefix = self.__class__.__name__.lower()
--> 109             name = prefix + '_' + str(K.get_uid(prefix))
    110         self.name = name
    111 

~/opt/anaconda3/envs/python/lib/python3.7/site-packages/keras/backend/tensorflow_backend.py in get_uid(prefix)
     72     """
     73     global _GRAPH_UID_DICTS
---> 74     graph = tf.get_default_graph()
     75     if graph not in _GRAPH_UID_DICTS:
     76         _GRAPH_UID_DICTS[graph] = defaultdict(int)

AttributeError: module 'tensorflow' has no attribute 'get_default_graph'