git bash commands with python or Gitpyhon

Question

So I have follwoing bash command which is part of a script, that will list just remote releases branches in a repo with no updates in more than 6 months

for b in $(git branch -r --merged | grep release) ; do echo "$(git log -1  --pretty=format:"%Cgreen%as" --before '7 month ago' $b) $b"; done

this will print :

2020-04-27 origin/release/1.1
2020-04-28 origin/release/1.2
2020-04-28 origin/release/1.3
2020-04-29 origin/release/1.4
2020-05-01 origin/release/1.5

The idea is that script will print. "Following merged branches are going to be removed: " and then execute the git push origin --delete {branch} to those branches

Now will like to achieve something similar in python since I´m starting with it...but not quite expert. Was trying with something like this but this doesn´t show the same output than bash:

import subprocess

def release_branches():
    result_release = subprocess.getoutput("git branch -r --merged | grep release ")
    return [b.strip() for b in result_release.split('\n')]
        

def branch_stale(branch):
    commits_since_seven = subprocess.getoutput(f"git log -1 --pretty=format:"\%Cgreen%as\"{branch} --before '7 month ago'")
    if commits_since_seven is not None:
        return True


for branch in release_branches():
    if branch_stale(branch):
        print (branch)

Also is this the best way with subprocess and catching output? or is it better to do it with GitPython instead? thank you!