Making a certificate authority (CA) with python cryptography

ca.py

from cryptography import x509
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.x509.oid import NameOID
import datetime
import uuid
one_day = datetime.timedelta(1, 0, 0)
private_key = rsa.generate_private_key(
    public_exponent=65537,
    key_size=2048,
    backend=default_backend()
)
public_key = private_key.public_key()
builder = x509.CertificateBuilder()
builder = builder.subject_name(x509.Name([
    x509.NameAttribute(NameOID.COMMON_NAME, u'openstack-ansible Test CA'),
    x509.NameAttribute(NameOID.ORGANIZATION_NAME, u'openstack-ansible'),
    x509.NameAttribute(NameOID.ORGANIZATIONAL_UNIT_NAME, u'Default CA Deployment'),
]))
builder = builder.issuer_name(x509.Name([
    x509.NameAttribute(NameOID.COMMON_NAME, u'openstack-ansible Test CA'),
]))
builder = builder.not_valid_before(datetime.datetime.today() - one_day)
builder = builder.not_valid_after(datetime.datetime(2018, 8, 2))
builder = builder.serial_number(int(uuid.uuid4()))
builder = builder.public_key(public_key)
builder = builder.add_extension(
    x509.BasicConstraints(ca=True, path_length=None), critical=True,
)
certificate = builder.sign(
    private_key=private_key, algorithm=hashes.SHA256(),
    backend=default_backend()
)
print(isinstance(certificate, x509.Certificate))

with open("ca.key", "wb") as f:
    f.write(private_key.private_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PrivateFormat.TraditionalOpenSSL,
        encryption_algorithm=serialization.BestAvailableEncryption(b"openstack-ansible")
    ))

with open("ca.crt", "wb") as f:
    f.write(certificate.public_bytes(
        encoding=serialization.Encoding.PEM,
    ))

gistfile1.txt

$ openssl asn1parse -in ca.crt
    0:d=0  hl=4 l= 801 cons: SEQUENCE          
    4:d=1  hl=4 l= 521 cons: SEQUENCE          
    8:d=2  hl=2 l=   3 cons: cont [ 0 ]        
   10:d=3  hl=2 l=   1 prim: INTEGER           :02
   13:d=2  hl=2 l=  16 prim: INTEGER           :7024B38452CF480FBA3E5DF5937A1B58
   31:d=2  hl=2 l=  13 cons: SEQUENCE          
   33:d=3  hl=2 l=   9 prim: OBJECT            :sha256WithRSAEncryption
   44:d=3  hl=2 l=   0 prim: NULL              
   46:d=2  hl=2 l=  36 cons: SEQUENCE          
   48:d=3  hl=2 l=  34 cons: SET               
   50:d=4  hl=2 l=  32 cons: SEQUENCE          
   52:d=5  hl=2 l=   3 prim: OBJECT            :commonName
   57:d=5  hl=2 l=  25 prim: UTF8STRING        :openstack-ansible Test CA
   84:d=2  hl=2 l=  30 cons: SEQUENCE          
   86:d=3  hl=2 l=  13 prim: UTCTIME           :151111141635Z
  101:d=3  hl=2 l=  13 prim: UTCTIME           :180802000000Z
  116:d=2  hl=2 l=  96 cons: SEQUENCE          
  118:d=3  hl=2 l=  34 cons: SET               
  120:d=4  hl=2 l=  32 cons: SEQUENCE          
  122:d=5  hl=2 l=   3 prim: OBJECT            :commonName
  127:d=5  hl=2 l=  25 prim: UTF8STRING        :openstack-ansible Test CA
  154:d=3  hl=2 l=  26 cons: SET               
  156:d=4  hl=2 l=  24 cons: SEQUENCE          
  158:d=5  hl=2 l=   3 prim: OBJECT            :organizationName
  163:d=5  hl=2 l=  17 prim: UTF8STRING        :openstack-ansible
  182:d=3  hl=2 l=  30 cons: SET               
  184:d=4  hl=2 l=  28 cons: SEQUENCE          
  186:d=5  hl=2 l=   3 prim: OBJECT            :organizationalUnitName
  191:d=5  hl=2 l=  21 prim: UTF8STRING        :Default CA Deployment
  214:d=2  hl=4 l= 290 cons: SEQUENCE          
  218:d=3  hl=2 l=  13 cons: SEQUENCE          
  220:d=4  hl=2 l=   9 prim: OBJECT            :rsaEncryption
  231:d=4  hl=2 l=   0 prim: NULL              
  233:d=3  hl=4 l= 271 prim: BIT STRING        
  508:d=2  hl=2 l=  19 cons: cont [ 3 ]        
  510:d=3  hl=2 l=  17 cons: SEQUENCE          
  512:d=4  hl=2 l=  15 cons: SEQUENCE          
  514:d=5  hl=2 l=   3 prim: OBJECT            :X509v3 Basic Constraints
  519:d=5  hl=2 l=   1 prim: BOOLEAN           :255
  522:d=5  hl=2 l=   5 prim: OCTET STRING      [HEX DUMP]:30030101FF
  529:d=1  hl=2 l=  13 cons: SEQUENCE          
  531:d=2  hl=2 l=   9 prim: OBJECT            :sha256WithRSAEncryption
  542:d=2  hl=2 l=   0 prim: NULL              
  544:d=1  hl=4 l= 257 prim: BIT STRING