fix: mpl_fix_backend for version<=3.1.3
fix: mpl_fix_backend for version<=3.1.3

* use() function must be done before importing matplotlib.pyplot
  https://matplotlib.org/3.1.3/tutorials/introductory/usage.html#what-is-a-backend

* UnboundLocalError: local variable 'matplotlib' referenced before assignment
  https://docs.python.org/3/faq/programming.html#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value

shmilee/gdpy3,shmilee/gdpy3,shmilee/gdpy3,shmilee/gdpy3