fix TypeError: use() got an unexpected keyword argument 'warn'

Author: AustinDeric

setup.py

@@ -10,7 +10,7 @@
 EXTENSIONS = [ext_1]
 
 setup(name='stl_tools',
-      version='0.4.0',
+      version='0.4.1',
       install_requires=[
         'numpy==1.18.1',
         'scipy',

stl_tools/text2png.py

@@ -1,6 +1,6 @@
 import os
 import matplotlib as mpl
-mpl.use('Agg', warn=False)
+mpl.use('Agg')
 import matplotlib.pyplot as plt