Models: add type defs, remove ambiguity of Base

To comply with mypy, type definitions have been added and the name Base which
before was using the factory declarative_base now use the decorator
as_declarative. Disable no-member for cls, pylint thinks its a reference to an
instance(a "self") of the class rather than a reference to the class.

type: ignore on Base.__tablename__ due to false-positive error,
see sqlalchemy/sqlalchemy2-stubs#127

caramel/models.py

@@ -3,8 +3,8 @@
 
 import sqlalchemy as _sa
 from sqlalchemy.ext.declarative import (
-    declarative_base as _declarative_base,
-    declared_attr as _declared_attr,
+    declared_attr,
+    as_declarative
 )
 import sqlalchemy.orm as _orm
 from zope.sqlalchemy import register
@@ -14,7 +14,7 @@
 import datetime as _datetime
 import dateutil.parser
 import uuid
-
+from typing import List
 
 X509_V3 = 0x2  # RFC 2459, 4.1.2.1
 
@@ -88,10 +88,11 @@ def _fkcolumn(referent, *args, **kwargs):
 register(DBSession)
 
 
+@as_declarative()
 class Base(object):
-    @_declared_attr
-    def __tablename__(cls):
-        return cls.__name__.lower()
+    @declared_attr  # type: ignore
+    def __tablename__(cls) -> str:
+        return cls.__name__.lower() # pylint: disable=no-member
 
     id = _sa.Column(_sa.Integer, primary_key=True)
 
@@ -108,10 +109,6 @@ def all(cls):
         return cls.query().all()
 
 
-# XXX: Newer versions of sqlalchemy have a decorator variant 'as_declarative'
-Base = _declarative_base(cls=Base)
-
-
 # XXX: not the best of names
 def init_session(engine, create=False):
     DBSession.configure(bind=engine)
@@ -135,10 +132,10 @@ class CSR(Base):
     orgunit = _sa.Column(_sa.String(_UB_OU_LEN))
     commonname = _sa.Column(_sa.String(_UB_CN_LEN))
     rejected = _sa.Column(_sa.Boolean(create_constraint=True))
-    accessed = _orm.relationship(
+    accessed: List["AccessLog"] = _orm.relationship(
         "AccessLog", backref="csr", order_by="AccessLog.when.desc()"
     )
-    certificates = _orm.relationship(
+    certificates: List["Certificate"] = _orm.relationship(
         "Certificate",
         backref="csr",
         order_by="Certificate.not_after.desc()",