Sqlintuitive

- replaced pytest.raises with assertRaises from unittest
- fixed primary key bug that generated invalid SQL

- converted test-cases into 'unittest.TestCase's
- added primary and foreign key constraits
- added unique contrait
- custom exceptions got a hierarchy

- added importing sub-modules recursivly in all \_\_init\_\_.py files

- added MySql db system with connect_to_dn(), close_connection(), get_cursor()
- MySQL db system got functions to process db actions
- renamed test files to order their execution
- create_table() got the option to use "IF NOT EXISTS"
- no longer directly inserting values into SQL statements -> added parameter for defining a placeholder (default: '?')
- introduced SQL generator select
- new requirement: mysql-connector-python

- introduced the following SQL generators:
- drop_table
- drop_db

- minimum python version set to 3.8 or newer