Make langchain compatible with SQLAlchemy<1.4.0

[mhwang-stripe]

Description

Currently SQLAlchemy >=1.4.0 is a hard requirement. We are unable to run from langchain.vectorstores import FAISS with SQLAlchemy <1.4.0 due to top-level imports, even if we aren't even using parts of the library that use SQLAlchemy. See Testing section for repro. Let's make it so that langchain is still compatible with SQLAlchemy <1.4.0, especially if we aren't using parts of langchain that require it.

The main conflict is that SQLAlchemy removed declarative_base from sqlalchemy.ext.declarative in 1.4.0 and moved it to sqlalchemy.orm. We can fix this by try-catching the import. This is the same fix as applied in #883.

(I see that there seems to be some refactoring going on about isolating dependencies, e.g. c87e9fb, so if this issue will be eventually fixed by isolating imports in langchain.vectorstores that also works).

Issue

I can't find a matching issue.

Dependencies

No additional dependencies

Maintainer

@hwchase17 since you reviewed #883

Testing

I didn't add a test, but I manually tested this.

1. Current failure:

langchain==0.0.305
sqlalchemy==1.3.24

python -i
>>> from langchain.vectorstores import FAISS
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/pay/src/zoolander/vendor3/lib/python3.8/site-packages/langchain/vectorstores/__init__.py", line 58, in <module>
    from langchain.vectorstores.pgembedding import PGEmbedding
  File "/pay/src/zoolander/vendor3/lib/python3.8/site-packages/langchain/vectorstores/pgembedding.py", line 10, in <module>
    from sqlalchemy.orm import Session, declarative_base, relationship
ImportError: cannot import name 'declarative_base' from 'sqlalchemy.orm' (/pay/src/zoolander/vendor3/lib/python3.8/site-packages/sqlalchemy/orm/__init__.py)

2. This fix:

langchain==<this PR>
sqlalchemy==1.3.24

python -i
>>> from langchain.vectorstores import FAISS
<succeeds>

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[eyurtsev]

@mhwang-stripe the pyproject file pins sqlalchemy current to a min version of 1.4. I"ll merge the code as is, but folks may end up with issues installing due to pip dependency resolution. But we're planning on making sqlalchemy an optional dependency at some point soon.