style: Fix flake8 errors and docstrings

Previous testing had ignored greentea source files when running flake8.
As a result, a number of style issues needed to be resolved. Along with
this, docstrings are updated, in google-style.

Author: wernerlewis

CONTRIBUTING.md

@@ -129,9 +129,14 @@ Example:
 
 ```python
 def make_squares(key, value=0):
-    """! Return a dictionary and a list...
-    @param value Value parameter with default value of zero (0)
-    @return Retruns tuple of 'd' stuff and 'l' stuff
+    """Return a dictionary and a list.
+
+    Args:
+        key: Key parameter.
+        value: Value parameter with default value of zero (0).
+
+    Returns:
+        Tuple of key:value dict and key, value list.
     """
     d = {key: value}
     l = [key, value]

setup.py

@@ -2,28 +2,28 @@
 # Copyright (c) 2021 Arm Limited and Contributors. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 #
-"""
-This module defines the attributes of the
-PyPI package for the mbed SDK test suite
-"""
-
+"""Package definition for PyPI."""
 import os
 from io import open
 from setuptools import setup, find_packages
 
-DESCRIPTION = "Greentea test suite is a collection of tools that enable automated testing on mbed-enabled platforms"
+DESCRIPTION = (
+    "Greentea test suite. A collection of tools that enable automated testing."
+)
 OWNER_NAMES = "Mbed team"
 OWNER_EMAILS = "[email protected]"
 
 repository_dir = os.path.dirname(__file__)
 
 
 def read(fname):
-    """
-    Utility function to cat in a file (used for the README)
-    @param fname: the name of the file to read,
-    relative to the directory containing this file
-    @return: The string content of the opened file
+    """Read the string content of a file.
+
+    Args:
+        name: the name of the file to read relative to this file's directory.
+
+    Returns:
+        String content of the opened file.
     """
     with open(os.path.join(repository_dir, fname), mode="r") as f:
         return f.read()

src/mbed_greentea/__main__.py

@@ -2,6 +2,7 @@
 # Copyright (c) 2021 Arm Limited and Contributors. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 #
+"""Main for the package."""
 from mbed_greentea.greentea_cli import main
 
 main()