acme: remove Client and BackwardsCompatibleClientV2 (#9356)

* acme: remove Client and BackwardsCompatibleClientV2

* remove ClientTestBase and some unused variables

* add ClientV2.get_directory

* tweak ToS callback code

* acme: update example to use ClientV2.get_directory

* simplify ToS callback further into one step

* further removal of acmev1-related code

- remove acme.client.ClientBase
- remove acme.mixins.VersionedLEACMEMixin
- remove acme.client.DER_CONTENT_TYPE
- remove various ACMEv1 special cases
- remove acme.messages.ChallengeResources.combinations

* remove .mixins.ResourceMixin, fields.resource, fields.Resource
and resource field from various .message classes.

* simplify acme.messages.Directory:

- remove Directory.register
- remove HasResourceType and GenericHasResourceType
- remove ability to look up Directory resources by anything other
  than the exact field name in RFC8555 (section 9.7.5)

* remove acme.messages.OLD_ERROR_PREFIX and support the old prefix

* remove acme.mixins

* reorder imports

* add comment to Directory about resource lookups

* s/new-cert/newOrder/

* get rid of `resource` sillyness in tests

* remove acmev1 terms-of-service support from directory

acme/examples/http01_example.py

@@ -163,7 +163,7 @@ def example_http():
     # Register account and accept TOS
 
     net = client.ClientNetwork(acc_key, user_agent=USER_AGENT)
-    directory = messages.Directory.from_json(net.get(DIRECTORY_URL).json())
+    directory = client.ClientV2.get_directory(DIRECTORY_URL, net)
     client_acme = client.ClientV2(directory, net=net)
 
     # Terms of Service URL is in client_acme.directory.meta.terms_of_service
@@ -215,8 +215,7 @@ def example_http():
     try:
         regr = client_acme.query_registration(regr)
     except errors.Error as err:
-        if err.typ == messages.OLD_ERROR_PREFIX + 'unauthorized' \
-                or err.typ == messages.ERROR_PREFIX + 'unauthorized':
+        if err.typ == messages.ERROR_PREFIX + 'unauthorized':
             # Status is deactivated.
             pass
         raise