Unpin sqlalchemy once issue is fixed

[albertvillanova]

Once the source issue is fixed:

• BUG: SQLAlchemy 2.0.0 creates incompatibility with Pandas read_sql (fails to execute) pandas-dev/pandas#51015

we should revert the pin introduced in:

• Pin sqlalchemy #5476

[lazarust]

@albertvillanova It looks like that issue has been fixed so I made a PR to unpin sqlalchemy!

[albertvillanova]

The source issue:

• ENH: Upgrade Pandas SQLAlchemy code to be compatible with 2.0+ syntax pandas-dev/pandas#40686

has been fixed:

• Make pandas/io/sql.py work with sqlalchemy 2.0 pandas-dev/pandas#48576

The fix was released yesterday (2023-04-03) only in pandas-2.0.0:

• https://github.com/pandas-dev/pandas/releases/tag/v2.0.0

but it will not be back-ported to pandas-1:

• Make pandas/io/sql.py work with sqlalchemy 2.0 pandas-dev/pandas#48576 (comment)

Also note that pandas-2.0.0 dropped support for Python 3.7:

• Drop support for Python 3.7 pandas-dev/pandas#41678
• DEPS: drop Python 3.7 and NumPy 1.17 from tests (#41678) pandas-dev/pandas#41989

Therefore, we cannot unpin sqlalchemy until we drop support for Python 3.7 (these Python users cannot use pandas-2).