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>

tests/scripts/scripts_path.py

@@ -0,0 +1,28 @@
+"""Add our Python library directory to the module search path.
+
+Usage:
+
+    import scripts_path # pylint: disable=unused-import
+"""
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+import sys
+
+sys.path.append(os.path.join(os.path.dirname(__file__),
+                             os.path.pardir, os.path.pardir,
+                             'scripts'))