Make langchain compatible with SQLAlchemy<1.4.0 (#11390)

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## 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 https://github.com/langchain-ai/langchain/pull/883.

(I see that there seems to be some refactoring going on about isolating
dependencies, e.g.
c87e9fb2ce,
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
https://github.com/langchain-ai/langchain/pull/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
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
python -i
>>> from langchain.vectorstores import FAISS
<succeeds>
```

libs/langchain/langchain/vectorstores/pgembedding.py

@@ -7,7 +7,12 @@ from typing import Any, Dict, Iterable, List, Optional, Tuple, Type
 import sqlalchemy
 from sqlalchemy import func
 from sqlalchemy.dialects.postgresql import JSON, UUID
-from sqlalchemy.orm import Session, declarative_base, relationship
+from sqlalchemy.orm import Session, relationship
+
+try:
+    from sqlalchemy.orm import declarative_base
+except ImportError:
+    from sqlalchemy.ext.declarative import declarative_base
 
 from langchain.docstore.document import Document
 from langchain.schema.embeddings import Embeddings

libs/langchain/langchain/vectorstores/pgvector.py

@@ -23,7 +23,12 @@ import numpy as np
 import sqlalchemy
 from sqlalchemy import delete
 from sqlalchemy.dialects.postgresql import UUID
-from sqlalchemy.orm import Session, declarative_base
+from sqlalchemy.orm import Session
+
+try:
+    from sqlalchemy.orm import declarative_base
+except ImportError:
+    from sqlalchemy.ext.declarative import declarative_base
 
 from langchain.docstore.document import Document
 from langchain.schema.embeddings import Embeddings