Support sqlalchemy >= 1.4.0

[aderk]

Is your feature request related to a problem? Please describe.
sqlalchemy 2.0 has breaking API changes that can be previewed in 1.4. The highest version of sqlalchemy supported by ptyhon-spanner-sqlalchemy is 1.3.23. Currently when using python-spanner-sqlalchemy, users must use the deprecated API, because 1.3.23 does not support the new API.

Describe the solution you'd like
python-spanner-sqlalchemy supports a version of sqlalchemy >= 1.4.0

Describe alternatives you've considered
Not sure

[skuruppu]

@aderk I think this is a reasonable request. Based on #105, @IlyaFaer started work on #123 as this was something we wanted to support but we then refocused our efforts on GA'ing the support that we had already built.

We will certainly consider continuing this work next year, just need to figure out priorities with respect to some other projects as well.

[IlyaFaer]

The PR which prepared a test suite for SQLAlchemy 1.4 is now merged. All the tests passed, so I guess SQLAlchemy 1.4 support is implemented.

Closing the issue, feel free to open in case something's acted up during migration to 1.4 version.

[aderk]

setup.py limits the version to < 1.4.0, can this version restriction be changed to allow 1.4.*?
https://github.com/googleapis/python-spanner-sqlalchemy/blob/main/setup.py#L24