Fix migration error in token due to missing userid field

Replace imports from `h.models` with local ORM classes. The ORM classes in
`h.models` assume the DB has the latest schema, which is not the case when
running migrations. The new code matches how other migrations work.

Fixes #8541

h/migrations/versions/8e3417e3713b_back_fill_the_token_user_id_column.py

@@ -4,18 +4,40 @@
 import re
 
 from alembic import op
-from sqlalchemy import select
+from sqlalchemy import Column, ForeignKey, Integer, UnicodeText, select
+from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker
 
-from h.models import Token, User
-
 revision = "8e3417e3713b"
 down_revision = "8fcdcefd8c6f"
 
 
 log = logging.getLogger(__name__)
 
 
+Base = declarative_base()
+
+
+class Token(Base):
+    __tablename__ = "token"
+
+    id = Column(Integer, primary_key=True)
+
+    # Legacy `userid` column.
+    userid = Column(UnicodeText())
+
+    # Replacement foreign key.
+    user_id = Column(Integer, ForeignKey("user.id"))
+
+
+class User(Base):
+    __tablename__ = "user"
+
+    id = Column(Integer, primary_key=True)
+    username = Column(UnicodeText(), nullable=False)
+    authority = Column(UnicodeText(), nullable=False)
+
+
 def split_userid(userid):
     return re.match(r"^acct:([^@]+)@(.*)$", userid).groups()