pin to numpy 1.19.5

Author: AustinDeric

setup.py

@@ -12,7 +12,7 @@
 setup(name='stl_tools',
       version='0.4.0',
       install_requires=[
-        'numpy==1.20.0',
+        'numpy==1.19.5',
         'scipy',
         'matplotlib'],
       description="Generate STL files from numpy arrays and text",