fixup http01_example.py (#10285)

it looks like https://github.com/certbot/certbot/pull/10098 introduced a
couple bugs into this file:

1.
[RSAPrivateKeys](https://cryptography.io/en/latest/hazmat/primitives/asymmetric/rsa/#cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey)
don't have a `public_bytes` method
2. `cryptography.x509` wasn't imported and
[load_pem_x509_certificate](https://cryptography.io/en/latest/x509/reference/#cryptography.x509.load_pem_x509_certificate)
takes bytes, not a string

i think avoiding this is unfortunately difficult as this file has no
tests, but it was useful for me just now when testing
https://github.com/certbot/certbot/pull/10283 so i wanted to fix it up

i also changed the script to initially create the account without an
email address as the fake@example.com email causes registration with
LE's staging server to fail early in execution

with the changes in this PR changes, if you:

1. change the value of
[DOMAIN](https://github.com/certbot/certbot/blob/0075104805e2e4fd9cf0e6ec69d80ab203e03655/acme/examples/http01_example.py#L57)
to a domain pointing at your machine
2. as root, activate your certbot dev environment, and run `python
acme/examples/http01_example.py `

it will fail late in the script with:
```
Traceback (most recent call last):
  File "/home/brad/certbot/acme/examples/http01_example.py", line 237, in <module>
    example_http()
    ~~~~~~~~~~~~^^
  File "/home/brad/certbot/acme/examples/http01_example.py", line 223, in example_http
    regr = client_acme.update_registration(
        regr.update(
    ...<3 lines>...
        )
    )
  File "/home/brad/certbot/acme/src/acme/client.py", line 101, in update_registration
    updated_regr = self._send_recv_regr(regr, body=body)
  File "/home/brad/certbot/acme/src/acme/client.py", line 373, in _send_recv_regr
    response = self._post(regr.uri, body)
  File "/home/brad/certbot/acme/src/acme/client.py", line 392, in _post
    return self.net.post(*args, **kwargs)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/brad/certbot/acme/src/acme/client.py", line 766, in post
    return self._post_once(*args, **kwargs)
           ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/brad/certbot/acme/src/acme/client.py", line 781, in _post_once
    response = self._check_response(response, content_type=content_type)
  File "/home/brad/certbot/acme/src/acme/client.py", line 630, in _check_response
    raise messages.Error.from_json(jobj)
acme.messages.Error: urn:ietf:params:acme:error:invalidContact :: The provided contact URI was invalid :: Unable to update account :: invalid contact: contact email has forbidden domain "example.org"
```
if you also change [this email
variable](https://github.com/certbot/certbot/blob/0075104805e2e4fd9cf0e6ec69d80ab203e03655/acme/examples/http01_example.py#L223)
to a valid email address, the script will run successfully

acme/examples/http01_example.py

@@ -27,11 +27,11 @@ Workflow:
 """
 from contextlib import contextmanager
 
+from cryptography import x509
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import rsa
 import josepy as jose
-import OpenSSL
 
 from acme import challenges
 from acme import client
@@ -70,7 +70,9 @@ def new_csr_comp(domain_name, pkey_pem=None):
     if pkey_pem is None:
         # Create private key.
         pkey = rsa.generate_private_key(public_exponent=65537, key_size=CERT_PKEY_BITS)
-        pkey_pem = pkey.public_bytes(serialization.Encoding.PEM)
+        pkey_pem = pkey.private_bytes(encoding=serialization.Encoding.PEM,
+                                      format=serialization.PrivateFormat.PKCS8,
+                                      encryption_algorithm=serialization.NoEncryption())
 
     csr_pem = crypto_util.make_csr(pkey_pem, [domain_name])
     return pkey_pem, csr_pem
@@ -168,11 +170,8 @@ def example_http():
 
     # Terms of Service URL is in client_acme.directory.meta.terms_of_service
     # Registration Resource: regr
-    # Creates account with contact information.
-    email = ('fake@example.com')
     regr = client_acme.new_account(
-        messages.NewRegistration.from_data(
-            email=email, terms_of_service_agreed=True))
+        messages.NewRegistration.from_data(terms_of_service_agreed=True))
 
     # Create domain private key and CSR
     pkey_pem, csr_pem = new_csr_comp(DOMAIN)
@@ -200,7 +199,7 @@ def example_http():
 
     # Revoke certificate
 
-    fullchain_com = x509.load_pem_x509_certificate(fullchain_pem)
+    fullchain_com = x509.load_pem_x509_certificate(fullchain_pem.encode())
 
     try:
         client_acme.revoke(fullchain_com, 0)  # revocation reason = 0