sqlalchemy: Explicit commit in transaction context manager causes error.

Describe the bug

when explicitly committing a transaction in a context manager, it throws an error, this seems to be caused by a _assert_active call in Transaction.__exit__.

this is implied to work via code examples in the docs and it looks like the code following the assert handles a a transaction being already committed and just returns early?

Expected behavior No error

To Reproduce

import sqlalchemy.orm

engine = sqlalchemy.create_engine(
    "postgresql://postgres:postgres@db:5432/postgres", 
    future=True,
)

with sqlalchemy.orm.Session(engine, expire_on_commit=False) as session:
    with session.begin():
        session.execute("SELECT 1")
        session.commit()

Error

Traceback (most recent call last):
  File "{snip}/test.py", line 10, in <module>
    session.commit()
  File "{snip}/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 937, in __exit__
    self._assert_active(deactive_ok=True, prepared_ok=True)
  File "{snip}/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 617, in _assert_active
    raise sa_exc.ResourceClosedError(closed_msg)
sqlalchemy.exc.ResourceClosedError: This transaction is closed

Versions.

• OS: linux
• Python: 3.9
• SQLAlchemy: 1.4.8
• Database: postgres
• DBAPI: psycopg2 or asyncpg