requirements: Bump sqlalchemy lower constraint

We've started using sqlalchemy features that were introduced in 1.4,
which means we need to bump our lower constraint. Do this now.

At some point, we probably want to dump all lower-constraint information
from these files since it's no longer maintainable without the
equivalent job. That's a task left to later, however.

Change-Id: I8629225eeb51d95264d8a3e4b719268bb1597f4f
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

requirements.txt

@@ -8,7 +8,7 @@ debtcollector>=1.2.0 # Apache-2.0
 oslo.i18n>=3.15.3 # Apache-2.0
 oslo.config>=5.2.0 # Apache-2.0
 oslo.utils>=3.33.0 # Apache-2.0
-SQLAlchemy>=1.2.0 # MIT
+SQLAlchemy>=1.4.0 # MIT
 sqlalchemy-migrate>=0.11.0 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 # these are used by downstream libraries that require