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

tests/unit_tests/llms/test_base.py

@@ -1,6 +1,10 @@
 """Test base LLM functionality."""
 from sqlalchemy import Column, Integer, Sequence, String, create_engine
-from sqlalchemy.orm import declarative_base
+
+try:
+    from sqlalchemy.orm import declarative_base
+except ImportError:
+    from sqlalchemy.ext.declarative import declarative_base
 
 import langchain
 from langchain.cache import InMemoryCache, SQLAlchemyCache