update reruns pytest flag

fixes error message:

error: ambiguous option: --rerun could match --reruns, --reruns-delay

Author: tony

setup.cfg

@@ -3,4 +3,4 @@ exclude = .*/,.tox,*.egg,tmuxp/_compat.py,tmuxp/__*__.py,
 select = E,W,F,N
 
 [tool:pytest]
-addopts = --rerun 5
+addopts = --reruns 5