FEA Stress plot in Python from 3 1D-arrays

I have 3 1D arrays (node x-coordinates, node y-coordinates and Von-Mises stress scalar) exported from an FEA solver.

I want to create 2D contour plots as shown below in Python:

Stress plot example

I have managed to create such plot as shown below: Stress plot result

import matplotlib as mpl
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.tri as tri

for orient in ['top', 'bot', 'side']: 
    x = []
    y = []
    z = []
    stress = []
    with open('data.txt') as file:
        for line in file:
            cur_line = line.split('\t')
            cur_x_old = cur_line[0]
            cur_y_old = cur_line[1]
            cur_z_old = cur_line[2]
            cur_s_old = cur_line[3]
            
            if cur_x_old == 'X Location (mm)':
                pass
            else:
                cur_x = cur_x_old.replace(",",".")
                cur_y = cur_y_old.replace(",",".")
                cur_z = cur_z_old.replace(",",".")
                cur_s = cur_s_old.replace(",",".")
                
                x.append(float(cur_x))
                y.append(float(cur_y))
                z.append(float(cur_z))
                stress.append(float(cur_s))
                
    stress = np.array(stress)
    x = np.array(x)
    y = np.array(y)
    z = np.array(z)
    
    levels=np.linspace(stress.min(), stress.max(), num=100)
    triang = tri.Triangulation(x, y)
    
    if orient == 'side':
        plt.figure(figsize = (max(x)/50, abs(min(y))/50))
        plt.tricontourf(triang, stress, cmap = 'jet', norm = mpl.colors.Normalize(0, 100), levels = levels, extend = 'max')
        plt.scatter(x, y, color = 'k')
    else:
        plt.figure(figsize = (max(x)/50, max(z)*2/50))
        plt.tricontourf(x, z, stress, cmap = 'jet', norm = mpl.colors.Normalize(0, 100), levels = levels)

My problem is that by triangulating the data, unwanted triangles are generated at the edge of the mesh (see Stress plot result). The black dots are the scatter plot from x and y coordinates. I want the colour plot to be only inside the boundaries of the grid. Is there a way to remove these unwanted triangles?