format_keys.py

#!/usr/bin/env python3

def chunks(l, n):
    """ Yield successive n-sized chunks from l.
    """
    for i in range(0, len(l), n):
        yield l[i:i+n]

# uncomment as per your usage
#pem_key = '-----BEGIN PUBLIC KEY-----\\n'
#pem_key = '-----BEGIN CERTIFICATE-----\\n'
pem_key = '-----BEGIN PRIVATE KEY-----\\n'

key = 'your certificate or key all in one line without \n'


for chunk in list(chunks(key, 64)):
  pem_key += chunk + '\\n'

# uncomment as per your usage
#pem_key += '-----END PUBLIC KEY-----'
#pem_key += '-----END CERTIFICATE-----'
pem_key += '-----END PRIVATE KEY-----'

print(pem_key)