from tikzplotlib import get_tikz_code not working + propsed single line solution

[Ntropic]

v10.1 has a problem in tikzplotlib_axes.py
from matplotlib.backends.backend_pgf import common_texification as mpl_common_texification
Leading to:
ImportError: cannot import name 'common_texification' from 'matplotlib.backends.backend_pgf'

I tshould be:
from matplotlib.backends.backend_pgf import _tex_escape as mpl_common_texification instead.

[nschloe]

Dup of #559.