backends: db: Avoid deprecated Table.count SQLAlchemy API.

[Apteryks]

Fixes #1035.

• gourmet/backends/db.py (RecData.fetch_len): Use the sqlalchemy.func.count selector to return the table length.

How Has This Been Tested?

Building the package with GNU Guix and ensuring the test suite passes + summarily testing the resulting gourmet executable.

[Apteryks]

I'm no sqlalchemy expert, so review carefully, but it at least passes the test suite.