Remove Python2 compatibility

[soulee-dev]

In this PR, we have removed the compatibility with Python 2 to streamline the code and leverage the features of Python 3 more fully. The specific changes include the following:

• Removed # -*- coding: utf-8 -*- from the top of Python files, as Python 3 assumes UTF-8 encoding by default.
• Eliminated conditional imports that checked sys.version_info to import different modules or packages based on the Python version.
• Deleted _compat.py file which was used to handle string type differences between Python 2 and 3.

Changes

1. Removal of UTF-8 encoding declarations

   Python 3 assumes UTF-8 encoding by default, so it's safe to remove # -*- coding: utf-8 -*- declarations from all files.

2. Unification of imports

   Removed conditional imports that were dependent on the Python version. Now, all imports are unified to be compatible with Python 3.

   For instance, the following conditional import:

   if sys.version_info < (2, 7):
       import unittest2 as unittest
   else:
       import unittest

   Has been updated to simply:

   import unittest

3. Deletion of _compat.py

   We removed the _compat.py file, which was primarily used to handle string type differences between Python 2 and 3. With this change, we are standardizing on the Python 3 string type.