postgres check: bump datadog_checks_base min version

Fix broken build due to old datadog_checks_base minimum version.

```
E   ImportError: cannot import name 'compute_exec_plan_signature' from 'datadog_checks.base.utils.db.sql' (/home/vsts/work/1/s/postgres/.tox/py38/lib/python3.8/site-packages/datadog_checks/base/utils/db/sql.py)
```

Follow-up to #8627

Depends on #8759

postgres/setup.py

@@ -28,7 +28,7 @@ def get_dependencies():
         return f.readlines()
 
 
-CHECKS_BASE_REQ = 'datadog-checks-base>=15.3.0'
+CHECKS_BASE_REQ = 'datadog-checks-base>=16.5.0'
 
 setup(
     name='datadog-postgres',