I think following code is making it bold-

import numpy as np   
import matplotlib.pyplot as plt    
plt.rcParams['text.usetex'] = True

fig, ax = plt.subplots(figsize=(6, 4))

point1 = [0, 1]
point2 = [np.sqrt(2), 0]
x_values = [point1[0], point2[0]]
y_values = [point1[1], point2[1]]

ax.plot(x_values, y_values, 'bo', linestyle="-")

# Annotated line in bold LaTeX
ax.annotate(r"\boldmath{$y = -\,\frac{1}{\sqrt{2}}x + 1$}", 
            xy=(0.2, 0.7), rotation=-35, fontsize=12)

plt.show()

My output-

Output of your code-