docs: use lambda over getconn func

[jackwotherspoon]

Update README and integration tests to be more pythonic by using lambda
over intermediate getconn function.

Reduces code required and is more readable.

Instead of using getconn:

def getconn() -> pymysql.connections.Connection:
    conn: pymysql.connections.Connection = connector.connect(
        "project:region:instance",
        "pymysql",
        user="my-user",
        password="my-password",
        db="my-db-name"
    )
    return conn

pool = sqlalchemy.create_engine(
    "mysql+pymysql://",
    creator=getconn,
)

We can simplify it using a lambda:

pool = sqlalchemy.create_engine(
    "mysql+pymysql://",
    creator=lambda: connector.connect(
        "project:region:instance",
        "pymysql",
        user="my-user",
        password="my-password",
        db="my-db-name"
    ),
)