Change TF ODT API tests to run on Python 3.7.

PiperOrigin-RevId: 421046167

Author: khanhlvg

research/object_detection/utils/visualization_utils.py

@@ -1294,7 +1294,7 @@ def cdf_plot(values):
     fraction_of_examples = (np.arange(cumulative_values.size, dtype=np.float32)
                             / cumulative_values.size)
     fig = plt.figure(frameon=False)
-    ax = fig.add_subplot('111')
+    ax = fig.add_subplot(1, 1, 1)
     ax.plot(fraction_of_examples, cumulative_values)
     ax.set_ylabel('cumulative normalized values')
     ax.set_xlabel('fraction of examples')
@@ -1321,7 +1321,7 @@ def add_hist_image_summary(values, bins, name):
   def hist_plot(values, bins):
     """Numpy function to plot hist."""
     fig = plt.figure(frameon=False)
-    ax = fig.add_subplot('111')
+    ax = fig.add_subplot(1, 1, 1)
     y, x = np.histogram(values, bins=bins)
     ax.plot(x[:-1], y)
     ax.set_ylabel('count')