update minimum python version for linux wheel building

Author: asottile

bin/build-manylinux-wheels

@@ -22,7 +22,7 @@ def check_call(*cmd):
 def main():
     os.makedirs('dist', exist_ok=True)
     with tempfile.TemporaryDirectory() as work:
-        pip = '/opt/python/cp37-cp37m/bin/pip'
+        pip = '/opt/python/cp38-cp38/bin/pip'
         check_call(
             'docker', 'run', '-ti',
             # Use this so the files are not owned by root