ensure single import per line

This adds the very small plugin flake8-import-single which
will prevent us from having an import with more than one symbol
on a line.

Flake8 by itself prevents this pattern with E401:

import collections, os, sys

However does not do anything with this:

from sqlalchemy import Column, text

Both statements have the same issues generating merge artifacts
as well as presenting a manual decision to be made.   While
zimports generally cleans up such imports at the top level, we
don't enforce zimports / pre-commit use.

the plugin finds the same issue for imports that are inside of
test methods.   We shouldn't usually have imports in test methods
so most of them here are moved to be top level.

The version is pinned at 0.1.5; the project seems to have no
activity since 2019, however there are three 0.1.6dev releases
on pypi which stopped in September 2019, they seem to be
experiments with packaging.  The source for 0.1.5
is extremely simple and only reveals one method to flake8
(the run() method).

Change-Id: Icea894e43bad9c0b5d4feb5f49c6c666d6ea6aa1

3 files changed, 9 insertions, 8 deletions

diff --git a/lib/sqlalchemy/orm/clsregistry.py b/lib/sqlalchemy/orm/clsregistry.py
index e5fff4a5e..c4d6c29eb 100644
--- a/lib/sqlalchemy/orm/clsregistry.py
+++ b/lib/sqlalchemy/orm/clsregistry.py
@@ -553,7 +553,8 @@ def _resolver(
 
     if _fallback_dict is None:
         import sqlalchemy
-        from sqlalchemy.orm import foreign, remote
+        from . import foreign
+        from . import remote
 
         _fallback_dict = util.immutabledict(sqlalchemy.__dict__).union(
             {"foreign": foreign, "remote": remote}
diff --git a/lib/sqlalchemy/orm/loading.py b/lib/sqlalchemy/orm/loading.py
index 7974d94c5..3d9ff7b0a 100644
--- a/lib/sqlalchemy/orm/loading.py
+++ b/lib/sqlalchemy/orm/loading.py
@@ -37,6 +37,8 @@ from .base import _RAISE_FOR_STATE
 from .base import _SET_DEFERRED_EXPIRED
 from .base import PassiveFlag
 from .context import FromStatement
+from .context import ORMCompileState
+from .context import QueryContext
 from .util import _none_set
 from .util import state_str
 from .. import exc as sa_exc
@@ -55,7 +57,6 @@ from ..util import EMPTY_DICT
 if TYPE_CHECKING:
     from ._typing import _IdentityKeyType
     from .base import LoaderCallableStatus
-    from .context import QueryContext
     from .interfaces import ORMOption
     from .mapper import Mapper
     from .query import Query
@@ -519,9 +520,6 @@ def load_on_pk_identity(
 
     assert not q._is_lambda_element
 
-    # TODO: fix these imports ....
-    from .context import QueryContext, ORMCompileState
-
     if load_options is None:
         load_options = QueryContext.default_load_options
 
diff --git a/lib/sqlalchemy/testing/fixtures.py b/lib/sqlalchemy/testing/fixtures.py
index 3b58052b8..a8bc6c50a 100644
--- a/lib/sqlalchemy/testing/fixtures.py
+++ b/lib/sqlalchemy/testing/fixtures.py
@@ -24,7 +24,12 @@ from .entities import ComparableEntity
 from .entities import ComparableMixin  # noqa
 from .util import adict
 from .util import drop_all_tables_from_metadata
+from .. import Column
 from .. import event
+from .. import func
+from .. import Integer
+from .. import select
+from .. import Table
 from .. import util
 from ..orm import DeclarativeBase
 from ..orm import events as orm_events
@@ -247,9 +252,6 @@ class TestBase:
     def trans_ctx_manager_fixture(self, request, metadata):
         rollback, second_operation, begin_nested = request.param
 
-        from sqlalchemy import Table, Column, Integer, func, select
-        from . import eq_
-
         t = Table("test", metadata, Column("data", Integer))
         eng = getattr(self, "bind", None) or config.db