Python fix bugs

[tamarinvs19]

Description

1. Fixed problem with mypy error (missed import os)
2. Used toAbsolutePath for all CLI arguments
3. Updated run_tests.py
4. Code refactoring

How to test

Manual tests

1. Run utbot python on unannotated function, check that mypy access right type (for example samples/classes/rename_self.py
2. Run utbot-cli-python with relative paths (to file under test, to output file, to python, to sys-paths), check that there are no one exception about findMethodByHeader

Self-check list

Check off the item if the statement is true. Hint: [x] is a marked item.

Please do not delete the list or its items.

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.