Import of `declarative_base` when SQLAlchemy <1.4 (#883)
In
[pyproject.toml](https://github.com/hwchase17/langchain/blob/master/pyproject.toml),
the expectation is `SQLAlchemy = "^1"`. But, the way `declarative_base`
is imported in
[cache.py](https://github.com/hwchase17/langchain/blob/master/langchain/cache.py)
will only work with SQLAlchemy >=1.4. This PR makes sure Langchain can
be run in environments with SQLAlchemy <1.4
