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>

.pylintrc

@@ -1,3 +1,6 @@
+[MASTER]
+init-hook='import sys; sys.path.append("scripts")'
+
 [BASIC]
 # We're ok with short funtion argument names.
 # [invalid-name]