Allow tests/scripts/foo.py to import from scripts

Allow Python scripts in tests/scripts to import modules located in the
scripts directory. To do this, use
```
import scripts_path # pylint: disable=unused-import
```

Declare the scripts directory to pylint and to mypy.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.mypy.ini

@@ -0,0 +1,3 @@
+[mypy]
+mypy_path = scripts
+warn_unused_configs = True