Support SQL Alchemy 2.0

Running gertty on a more modern OS like Fedora 38, you get the following
error:

sqlalchemy.exc.InvalidRequestError: The 'sqlalchemy.orm.mapper()'
function is removed as of SQLAlchemy 2.0. Use the
'sqlalchemy.orm.registry.map_imperatively()` method of the
``sqlalchemy.orm.registry`` class to perform classical mapping.

This updates Gertty to accomodate the changes in SQL Alchemy 2.0.

Change-Id: I793e997d767c8419a0baaac9a074d6997b834a7c

gertty/db.py

@@ -25,12 +25,13 @@ import six
 import sqlalchemy
 from sqlalchemy import create_engine, MetaData, Table, Column, Integer, String, Boolean, DateTime, Text, UniqueConstraint
 from sqlalchemy.schema import ForeignKey
-from sqlalchemy.orm import mapper, sessionmaker, relationship, scoped_session, joinedload
+from sqlalchemy.orm import registry, sessionmaker, relationship, scoped_session, joinedload
 from sqlalchemy.orm.session import Session
 from sqlalchemy.sql import exists
 from sqlalchemy.sql.expression import and_
 
 metadata = MetaData()
+mapper_registry = registry()
 project_table = Table(
     'project', metadata,
     Column('key', Integer, primary_key=True),
@@ -713,8 +714,8 @@ class Check(object):
         self.created = created
         self.updated = updated
 
-mapper(Account, account_table)
-mapper(Project, project_table, properties=dict(
+mapper_registry.map_imperatively(Account, account_table)
+mapper_registry.map_imperatively(Project, project_table, properties=dict(
     branches=relationship(Branch, backref='project',
                           order_by=branch_table.c.name,
                           cascade='all, delete-orphan'),
@@ -740,16 +741,16 @@ mapper(Project, project_table, properties=dict(
                               order_by=change_table.c.number,
                           ),
 ))
-mapper(Branch, branch_table)
-mapper(Topic, topic_table, properties=dict(
+mapper_registry.map_imperatively(Branch, branch_table)
+mapper_registry.map_imperatively(Topic, topic_table, properties=dict(
     projects=relationship(Project,
                           secondary=project_topic_table,
                           order_by=project_table.c.name,
                           viewonly=True),
     project_topics=relationship(ProjectTopic),
 ))
-mapper(ProjectTopic, project_topic_table)
-mapper(Change, change_table, properties=dict(
+mapper_registry.map_imperatively(ProjectTopic, project_topic_table)
+mapper_registry.map_imperatively(Change, change_table, properties=dict(
         owner=relationship(Account),
         conflicts1=relationship(Change,
                                 secondary=change_conflict_table,
@@ -787,7 +788,7 @@ mapper(Change, change_table, properties=dict(
                                      order_by=(approval_table.c.category,
                                                approval_table.c.value))
         ))
-mapper(Revision, revision_table, properties=dict(
+mapper_registry.map_imperatively(Revision, revision_table, properties=dict(
         messages=relationship(Message, backref='revision',
                               cascade='all, delete-orphan'),
         files=relationship(File, backref='revision',
@@ -798,9 +799,9 @@ mapper(Revision, revision_table, properties=dict(
                             cascade='all, delete-orphan'),
 
         ))
-mapper(Message, message_table, properties=dict(
+mapper_registry.map_imperatively(Message, message_table, properties=dict(
         author=relationship(Account)))
-mapper(File, file_table, properties=dict(
+mapper_registry.map_imperatively(File, file_table, properties=dict(
        comments=relationship(Comment, backref='file',
                              order_by=(comment_table.c.line,
                                        comment_table.c.created),
@@ -812,20 +813,20 @@ mapper(File, file_table, properties=dict(
                                              comment_table.c.created)),
        ))
 
-mapper(Comment, comment_table, properties=dict(
+mapper_registry.map_imperatively(Comment, comment_table, properties=dict(
         author=relationship(Account)))
-mapper(Label, label_table)
-mapper(PermittedLabel, permitted_label_table)
-mapper(Approval, approval_table, properties=dict(
+mapper_registry.map_imperatively(Label, label_table)
+mapper_registry.map_imperatively(PermittedLabel, permitted_label_table)
+mapper_registry.map_imperatively(Approval, approval_table, properties=dict(
         reviewer=relationship(Account)))
-mapper(PendingCherryPick, pending_cherry_pick_table)
-mapper(SyncQuery, sync_query_table)
-mapper(Hashtag, hashtag_table)
-mapper(Server, server_table, properties=dict(
+mapper_registry.map_imperatively(PendingCherryPick, pending_cherry_pick_table)
+mapper_registry.map_imperatively(SyncQuery, sync_query_table)
+mapper_registry.map_imperatively(Hashtag, hashtag_table)
+mapper_registry.map_imperatively(Server, server_table, properties=dict(
     own_account=relationship(Account)
     ))
-mapper(Checker, checker_table)
-mapper(Check, check_table, properties=dict(
+mapper_registry.map_imperatively(Checker, checker_table)
+mapper_registry.map_imperatively(Check, check_table, properties=dict(
     checker=relationship(Checker)))