BLD: certifi is not a run-time dependency

Closes matplotlib#18337

Author: tacaswell

setup.py

@@ -287,7 +287,6 @@ def build_extensions(self):
         "numpy>=1.15",
     ],
     install_requires=[
-        "certifi>=2020.06.20",
         "cycler>=0.10",
         "kiwisolver>=1.0.1",
         "numpy>=1.16",