Use relative imports when importing other modules in the same directory

We were using absolute imports under the assumption that the /scripts
directory is in the path. This worked in normal use because every one of our
Python scripts either were in the /scripts directory, or added the /scripts
directory to the module search path in order to reference mbedtls_dev.
However, this broke things like
```
python3 -m unittest scripts/mbedtls_dev/psa_storage.py
```

Fix this by using relative imports.

Relative imports are only supposed to be used inside a package (Python
doesn't complain, but Pylint does). So make /scripts/mbedtls_dev a proper
package by creating __init__.py.

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

scripts/mbedtls_dev/test_data_generation.py

@@ -29,8 +29,8 @@ import re
 from abc import ABCMeta, abstractmethod
 from typing import Callable, Dict, Iterable, Iterator, List, Type, TypeVar
 
-from mbedtls_dev import build_tree
-from mbedtls_dev import test_case
+from . import build_tree
+from . import test_case
 
 T = TypeVar('T') #pylint: disable=invalid-name