fix latex font issues in py version >=39

added a condition to fix the missing re._alphanum_str() for python >=39. It seems for newer version nothing needs to be done. The codes below works fine with the proposed fix on fedora33 w/ 3.9.9.

from matplotlib import rc
from opal.opal import load_dataset
import matplotlib.pyplot as plt
import numpy as np

FontSize=22
FontSizeLabelAxis=22
rc('xtick',labelsize=FontSize)
rc('ytick',labelsize=FontSize)
rc('text', usetex=True)

x=np.linspace(-10,10,100)
plt.plot (x,x**2)
plt.xlabel (r'$x$ ', fontsize=FontSize)
plt.ylabel (r'$y=x^2$ ', fontsize=FontSize)
plt.tight_layout()
plt.show()

fixes #101 (closed)